310 



ZOOLOGY 



SECT. 



b- 



central cavity is enclosed by the latter. The ectoderm layer, which is devoid 

 of cell-limits, secretes a cuticular membrane investing the embryo ; then a 

 second membrane is formed within the first, and a third within the second ; 



the embryo thus comes to be enclosed in a triple 

 case (Fig. 259), which differs from an egg-shell in 

 being formed by the developing ectoderm. At 

 what will become the anterior end chitinoid hooks 

 appear. 



At about this period the embryo is born, and 

 reaching the intestine of the host, is extruded with 

 its faeces. Its further development depends upon 

 its being swallowed by an intermediate host, which, 

 in the case of E. gigas of the Pig is a maggot, the 

 larva of a Beetle, Cetonia aurata. The Echino- 

 rhynchi of fresh-water Fish have for their inter- 

 mediate host certain small fresh-water Crustacea 

 belonging to the genera Gammarus and Asellus. 



Having reached the intestine of the intermediate 

 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 development. 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 oiater layer to an epithelium, from which the 

 body-muscles arise ; the cavity formed by the 

 splitting of the endoderm is the crelome. Part of 

 the proboscis and its sheath are also of endodermal 

 origin. The ectoderm gives rise to the protoplasmic 

 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 intermediate host is eaten by the permanent 

 host. 



'; 



CLASS III. CH^TOGNATHA. 



The present group, like that just discussed, is a 

 very small one, containing only three genera 

 (Sagitta, Spadella and Krohnia) of curious arrow- 

 shaped worms, all but one species of which are 

 pelagic. 



External Characters. The body (Fig. 260) is 



FIG 1^9. Egg of Eciuno- elongated and nearly cylindrical, and is divided into 

 rhynchus acus, en- , f , . .; 7 /, , , v. . , j . , 



dused in a triple case ; head, trunk: and tail, the head being marked on by 



highly magnified, a. endo- its somewhat rounded form, while the junction of 



cmtfnJn*- ho e okt le F'ro m tl>unk and tail is .indicated by the ventrally placed 

 Shipley, after Hamann.) anus (a). The tail bears a horizontal expansion, or 



caudal fin (s. fl.), and there are also horizontal 

 lateral fins (jl.) 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 deric epithelium (Fig. 261, d. epthm), which, 

 instead of being sj'ncytial as in the two preceding classes, is formed of several 

 layers of epithelial cells. Next comes a delicate basement membrane, and then 



