Open Access

Taisoo Park

• Epilabidocera amphitrites is one of the most common copepods in the deep waters adjacent to Friday Harbor and shows characteristic swarming behavior in the surface film of the water from later spring through early summer. That the swarms are composed mainly, up to 99 %, of adult males appears to be due to difference in phototaxis to a weak light. This species, at least in copepodid stages, is omnivorous, but seems to prefer an animal diet rather than diatoms. Reproduction takes place continuously from early spring through autumn. The external anatomy of both the female and male has been described in detail. The cuticle forming the arthrodial membrane and the lining of the esophagus, hindgut, and hypostomal and labral troughs appears to be of the same nature throughout, consisting of a single stratum. The cuticle on the general body surface, however, consists of two main strata. The endoskeletal structures consist of two categories, the endoskeleton proper and the endoskeletal tendons. The former involves apodemes and apophyses. Of these the major ones are described in detail. The latter consist of two median tendinous endosternites in the « head », four pairs of ventral intersegmental thoracic tendons, and a pair of dorsal longitudinal tendons in the metasome. The endosternites are well developed, serving as origins for dilators to the atrium oris and esophagus and also for a number of extrinsic muscles to the head appendages. The skeletomusculature may be divided into longitudinal trunk and limb muscles. The paired dorsal and ventral longitudinal trunk muscles in the metasome extend, respectively, from the levels of the cervical groove and the post-maxillulary apodeme to the end of the metasome. The longitudinal trunk muscles in the urosome origate at the anterior end and run most of its length. They are arranged as paired dorsal and ventral groups and a pair of lateral muscles. The extrinsic limb muscles are described in detail. They originate either from the lateral to dorsal exoskeleton or from the endosternites. The digestive tract starts with the atrium oris in the oral cone, followed by the mouth proper, esophagus, midgut, and finally by the hindgut which opens as the anus at the end of the urosome. The oral cone consisting of the three lobed labrum and the paired paragnaths has a longitudinal groove, the oral groove, which is covered ventrally by the spinulose setae of the maxillae and laterally by the gnathobasal endites of the maxillules, these together forming an effective feeding apparatus. The midgut is produced anteriorly into a diverticulum which is higly secretory. In the middle portion of the midgut the epithelial cells are highly vacuolated. As they pass through this vacuolated region the gut contents are cemented into fecal pellets by a mucous secretion and they acquire a peritrophic membrane. There is a strong valve between the midgut and the hindgut. Peristalsis in the midgut is irregular but powerful and primarily in the reverse direction. The circulatory system involves a single heart, enclosed in a large pericardial space, and an anteriorly directed aorta terminating in an anterodorsal aortic sines. The latter communicates through three paires of openings with the sinuses in the head, which are in turn continuous with the perivisceral cavity, from which blood is returned to the pericardium. The heart has the form of a flask with an aortic valve at the tapered anterior end and a posterior ostium. The aortic wall is continued posteriorly over the heart and wraps around the anterior three-fifths as an outer membrane. This outer membrane is extended dorsally at three places to attach the heart to the dorsal exoskeleton; and it is also drawn out ventrally to form the anterior and lateral walls of the pericardium. These walls are continuous with the pericardial floor which seals the pericardia! cavity from the perivisceral cavity. The heart-beat and the blood flow through the system have been discussed. The excretory system consists of a pair of maxillary glands, each comprising a coelomic end-sac, a coelomic secretory tubule and an ectodermal excretory duct. The end-sac communicates with the tubule through a valvular opening. Antennary glands are not gound either in the nauplius stage or in the adult. The male reproductive system consists of a single testis and a single genital duct which is divided into four differentiated sections, the vas deferens, the seminal vesicle, the spermatophore sac, and the ductus ejaculatorius. The vas deferens is a thick-walled glandular tube secreting the various constituents of the spermatophore. The seminal vesicle serves mainly as a reservoir for the various components of a definitive spermatophore, and it is here that these take up their final positions. The spermatophore sac is highly glandular and is mainly responsible for formation of the coupling apparatus of the spermatophore. The spermatophore is not open directly to the outside but is connected with a canal system in the coupling apparatus. When transferred to the female genital segment at copulation, the central secretion of the spermatophore is discharged through the canal system of the coupling apparatus to glue down the spermatophore. A duct through which the spermatozoa can pass from the spermatophore to the spermathecae of the female appears to be formed later by an action of the female, possibly secretion of an enzyme or lysin. The discharge of the contents of the spermatophore is effected by swelling of Q-spermatozoa in the distal region of the spermatophore. The functional spermatozoa are spherical or polygonal and nonmotile. The female reproductive system consists of a single ovary, two oviducts, each with several diverticula, leading to the paired opnenings into the vaginal vacity, a pair of spermathecae and a pair of glands which open into the oviducts. In the mature female the oviducts are wide and sac-like, expanded by growing oocytes. However, the last portion of the oviduct is usually empty of eggs and is highly secretory. The oldest oocytes in the oviducts are usually at the metaphase of the first maturation division. The evidence points to the conclusion that the eggs are laid in this stage, and they are fertilized when they pass through the vaginal cavity. Oogenesis has been studied in detail. There are two periods of yolk formation: the first immediately after the dispersion of the mitochondrial bodies and the second in the last phase of the oocyte growth when the vacuoles in the cytoplasm are gradually replaced by yolk. Two dorsal ocelli, in the copepodid stages, are placed dorsolaterally against the exoskeleton and highly developed, each with a perfectly spherical, cuticular lens, while a single ventral ocellus remains unspecialized through the copepodid stages. Each dorsal ocellus proper is suspended in the head sinus by several connective tissue stands in addition to an aye muscle and consists of a large, syncytial pigmented cup occupied by a cellular sphere which is composed of 9 retinular and 4 crystalline cells. Each of the 9 retinular cells gives off an axon which leaves the ocellar cup at one of three places to proceed to the nauplius eye center in the protocerebrum. The ventral ocellus consists of two multinucleated pigmented cells, a cup-shaped tapetum, 6 retinular cells and about 8 conjunctival cells. Each of the 6 retinular cells sends an axon which loops over the posterior rim of the ocellar cup in common with the others to course to the nauplius eye center in the protocerebrum. The ventral ocellus is innervated by two afferent nerve fibers. There is also found a pair of conspicuous nerve fibers, possibly afferent, associated with the dorsal and ventral ocelli. A pair of accessory retinular groups, each consisting of three retinular cells, is found posterior to the dorsal ocelli. Three efferent aXOl1S from each group form a nerve running to the nauplius eye center in the protocerebrum. A pair of frontal organs, each innervated by a frontal nerve, lies in the anterior end of the head. The frontal nerves can be traced up to a pair of neuropiles immerdiately ventral to the nauplius eye center in the proto cerebrum. A pair of suprafrontal nerves branched off from the frontal nerves is found to innervate a pair of sensory filaments, the suprafrontal sensiIla, at the lower anterior end of the head. The central nervous system, consisting of a well developed brain connected by massive circumesophageaI connectives to the ventral nerve cord, has been described in detail. The ganglion cells are found throughout the nerve cord, and they are arranged into ganglia in the thoracic segments bearing the swimming legs. The stomatogastric nervous system has two pairs of labral and a single gastric ganglia. The medial pair of the labral ganglia forms anteriorly a single ganglion which is connected to the brain by three small nerves. The giant fiber system, consisting of giant motor fibers and giant interneurons, has been studied in detail, and it appears to constitute the effector portion of an escape reflex. The cutaneous glands opening through small pores in the cuticle of the metasome, urosome, and the appendages have been described. Chromatophores, unicellular or syncytial with several nuclei, are scattered deep in the body and are responsible for the metachrosis.