THE ACANTHOCEPHALA. 553 



spicula or skeletons of Forarninifera^ to itself, and thus be- 

 comes prov'ided with an adventitious skeleton, the nature of 

 which varies in different species, but is constant for each. 

 Reproduction is effected by ova, which are said to be modi- 

 fied cells of the endoderm. In Gastrophysenia, the endo- 

 derm of the innermost chamber alone gives rise to ova. The 

 place of development of the spermatozoa has not been made 

 out. 



Yelk-division is complete and regular, and gives rise to a 

 vesicula morula (arcliiblastula of Haeckel), each cell of which 

 is provided with a flagellate cilium. A gastrula arises by in- 

 vagination, but the final stages of development have not 

 been made out. 



As Haeckel points out, the Thysemaria are obviously re- 

 lated, on the one hand to the Porifera^ and on the other to 

 the Ccelenterata ; in fact, they very nearly represent the 

 morphological common plan of which these two groups are 

 modifications. 



The Acaxthocephala. — In their sexual state the para- 

 sites which constitute the genus Echinorhynchus inhabit the 

 various classes of the Vertehrata, while they are found in 

 the Invertehrata only in a sexless condition. 



The Echinorhynchus of the Flounder (Fig. 157), the 

 structure of which may serve as an illustration of that of the 

 group, inhabits the rectum of that fish, which it pierces in 

 such a manner that the anterior extremity or head projects, 

 inclosed within a cyst, upon the peritoneal surface, while the 

 body hangs freely into the cavity of the intestine. Where 

 the worm traverses the wall of the rectum it presents a 

 much constricted neck (Fig. 157,/"). It would appear that, 

 eventually, the Echinorhynchi completely pass out of the 

 intestine, as they are found inclosed in detached cjsts lying 

 in the peritoneal cavity. The anterior extremity of the Echi- 

 norhynchus is produced into a short cylindrical proboscis, 

 covered with many rows of recurved hooks, and, behind this, 

 it forms a dilatation, in which the integument and the mus- 

 cular coat are separated by a considerable interval. The 

 body, behind the constricted neck, which separates it from 

 this anterior dilatation, has a thick, yellowish outer wall, 

 between which and the inner muscular tunic lies a system, of 

 vessels, consisting of two longitudinal trunks, connected by 

 a network of anastomosing canals. 



These canals do not appear to possess distinct walls, nor 

 24 



