310 



ZOOLOGY 



SECT. 





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 foeces. 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 Echinorhynchi of 

 fresh-water Fish have for their intermediate host 

 certain small fresh-water Crustacea belonging to 

 the genera Gammarus and Axellut. 



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 in- 

 testine or makes its way into the coelome ; in 

 either case it soon begins to undergo further de- 

 velopment. The endoderm, hitherto a solid mass 

 of cells, undergoes a process of splitting, be- 

 coming 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 an 

 epithelium, from which the body-muscles arise ; 

 the cavity formed by the splitting of the endo- 

 derm is the ccelome. 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. 



-QV 



-ovci 



a- 



-ho 



FIG. 255. Sagitta hexaptera, 

 from the ventral aspect, a. 

 anus ; bg. ventral ganglion ; 

 d. intestine ; jl. lateral fins ; 

 ho. testis ; in. mouth; ov. ovary; 

 ovd. oviduct ; sc. cesophageal 

 connective ; sb. vesicula semin- 

 alis ; s. Jl. tail fin; *//, tail- 

 cavity;^, spermiduct. (From 

 Lang's Coiii/Hirti.tit-e Anatomy, 

 after Hertwig.) 



preceding classes, is formed of 

 delicate basement membrane, 



CLASS III, CH^ETOGNATHA. 



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. 255) 

 is elongated and nearly cylindrical, and is divided 

 into head, trunk, and tail, the head being marked 

 off by its somewhat rounded form, while the junc- 

 tion of trunk and tail is indicated by the ventrally 

 placed anus (a). The tail bears a horizontal ex- 

 pansion, or caudal fin (.s. Jl.}, and there are also 

 horizontal lateral fin* (#. ) 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. 256, d. eptlun], 

 which, instead of being syncytial as in the two 

 several layers of epithelial cells. Next comes a 

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



