VI 



PH YLU M NEMATHELMINTHES 



293 



being .swallowed by an intermediate host, which, in the case of E. gigas of the 



Pig is a maggot, the larva of a Beetle, Cefonia aurctfn. 

 fresh-water Fish have for their intermediate host 

 certain small fresh-water Crustacea belonging to 

 the genera Gammarus and Aw-tluN. 



Having reached the intestine of the inter- 

 mediate host, the chitinoid embryonic membranes 

 are dissolved by its digestive juices, and the 

 embryo either fixes itself to the wall of the 

 intestine or makes its way into the ccelome ; in 

 either case it soon begins to undergo further de- 

 velopment. The endoderm, hitherto a solid 

 mass of cells, undergoes a process of splitting, 

 becoming divided into an outer layer in contact 

 with the ectoderm and a solid central axis. The 

 latter gives rise to the reproductive organs and 

 the suspensory ligament, the outer layer to a 

 ccelomic epithelium, from which the body- 

 muscles arise ; the cavity formed by the splitting 

 of the endoderm is the ccelome. Part of the 

 proboscis and its sheath are also of endodermal 

 origin. The ectoderm gives rise to the proto- 

 plasmic layer of the body-wall, to the whole 

 system of vessels, and to the lemnisci. The 

 larval cuticle is thrown off and a new one 

 formed. The larva reaches adult proportions 

 and attains sexual maturity only if the inter- 

 mediate is eaten by the permanent host. 



The Echinorhynchi of 



-til 



efC- 



\ 



n 



-ov 



-d 



G.' 



OVft 



CLASS III. CILETOGNATKA. 



The present group, like that just discussed, 

 is a very small one, containing only two genera 

 (Sriijittn and S2JCidella) of curious arrow-shaped 

 worms, all but one species of which are pelagic. 



External Characters. The body (Fig. 239) 

 is elongated and nearly cylindrical, and is divided 

 into head, trunk, and fed/, the head being marked 

 off by its somewhat rounded form, while the 

 junction of trunk and tail is indicated by the 

 ventrally placed anus (a). The tail bears a hori- 

 zontal expansion, or ex tidal _fi'n (x v /f. ), ami there 

 are also horizontal latt.ral fin* (ji.) a single pair 

 in Spadella, two pairs in Sagitta. 



Body-wall. There is no cuticle, but the 

 outer layer of the body-wall is formed by an 

 epidermis or cleric epithelium (Fig. 240, d.epthm.}, 

 which, instead of being syncytial as in the two 

 preceding classes, is formed of several layers of 

 epithelial cells. Next comes a delicate hn.-n.mi nf 

 membrane, and then a layer of muscles (m.), the 

 fibres of which are striated and disposed longi- 

 tudinally in four bands two dorso-lateral and 

 two ventro-lateral an arrangement which recalls 

 that of the corresponding layer in Xematoda. 



Beneath the muscle comes a delicate layer of c-xlomic epithelium (ccel. <./>thm). 



Enteric Canal. The mouth (Fig. 239, m.) is a longitudinal slit-like aperture 



on the ventral surface of the head ; on either side of it are several sickle-shaped 



;. 23D. Sagitta hexaptera, 



from the ventral aspect, a. 

 anus ; li,i. ventral ganglion ; 

 <i. intestine; ri. lateral fins; 

 ho. testis; m. mouth ; oc. ovary; 

 oc<l. oviduct ; .<". < esophageal 

 connective ; xh. vesieula semin- 

 alis ; $.rt. tail fin : .-/. spermi- 

 ihu-t. (From Lung's Comparat- 

 ive Aiiatoiiiii, after Hertwig.) 



