186 



INVERTEBRATE MORPHOLOGY. 



Echinodera are not to be considered as being truly metameric, indications 

 of metamerism wiiich are found being altogether secondary and without 

 phylogenetic significance. Until, however, something has been ascertained 

 regarding their embryological history nothing can 

 be positively stated as to their affinities. It is 

 worthy of notice, however, that in some particulars 

 '^WM'^Wf^ they resemble the Gastrotricha, and it is not im- 



probable that their nearest allies are to be found 

 in that order, which on its part is related to th& 

 Kotifera (see p. 189) . 



Class Ch^itognatha. 



The Chsetognatha constitute a small 

 group of forms separable into two genera, 

 Sagitta and Spaddla. All the members 

 of the group are marine, and are elonga- 

 ted in form, the sides of the body being 

 furnished with one or two pairs of lateral 

 expansions or fins, to which is added a 

 caudal fin. The anterior portion of the 

 body is somewhat enlarged so as to form 

 a head, and on either side of the mouth 

 are a number of strong chitinous bristles 

 movable by means of special muscles 

 and serving the purpose of jaws. 



The ectoderm consists of several 

 layers of flattened cells giving rise in the 

 head region by secretion to chitinous 

 plates which serve for the attachment 

 of the muscles which move the jaw- 

 bristles. Both the lateral and the caudal 

 fins are ectodermal expansions consisting 

 of a homogeneous lamella covered by 

 one or two layers of ectodermal cells. 

 They possess no muscle-fibres and are 

 passive in locomotion, which is per- 

 formed by the contraction of the longitudinal muscles 

 producing rapid lateral movements of the posterior part of the 

 body. The genus Sagitta possesses two lateral fins, while in 

 SpadeUa (Fig. 92) but one large one is present. Below the 



Pig. 92. — Spadella cepJi- 



aloptera (after Hert- 



wiq). 

 ce = cerebral ganglion. 



i = latestiue. 

 = olfactory organ. 

 oe = eye. 

 ov = ovary. 



t = testis. 



