NO. II ECHINODERMS AS ABERRANT ARTHROPODS CLARK 1 3 



thin mantle without muscles and containing only an ovary, without 

 generative ducts, testis or nerve ganglion. During development the 

 visceral mass disintegrates so that at the time of hatching the mantle 

 contains a great number of cypris larvae ready to emerge. The 

 escape of the larvae is contemporaneous with, or soon followed by, 

 a moult of the host. The empty shells of the external sacs are carried 

 away with the cast skin, and the terminal swellings of the root system 

 emerge as a new crop of external sacs. 



In Peltogaster the body has an elongated sausage shape, with the 

 mantle opening at one end, and is attached by the peduncle about the 

 middle of its length. The mesentery is longitudinal on the proximal 

 side (next the peduncle). 



In Sacculina the w^hole body is flattened in the plane of the mesen- 

 tery and has assumed a secondary and superficial bilateral symmetry 

 about a plane at right angles to this and coinciding with the median 

 plane of the host. In other genera, such as Lerncoodisciis and Tri- 

 angulus, the symmetry becomes still more complicated. 



In Clistosaccus and in Sylon the genital organs are impaired. 



The peduncle perforates the integument of the host and gives off 

 on the inside the absorption roots which, in the case of Sacculina, 

 penetrate into all the organs of the host with the exception of the 

 gills and heart, and extend to the terminal segments of the legs and 

 into the antennules and eye stalks. In Duplorbis, in which the root 

 system appears to be absent, the peduncle is hollow, its cavity com- 

 municating with the closed mantle cavity and opening at the other 

 end into the body cavity (h?emoccele) of the host. 



Apart from a single nervous ganglion (absent in Thompsonia) 

 which lies close to the mesentery near the female genital openings, 

 the only organs present are those of the generative system. 



SphcBTOthylacus is parasitic on a simple ascidian (Polycarpa), liv- 

 ing attached by ramifying roots to the inner wall of the branchial 

 sac. The globular body is enclosed in a mantle which has a small 

 opening. There are no appendages, but there is a complete alimen- 

 tary canal with mouth and anus, the latter near the mantle opening. 



The two known species of Sarcotaces live embedded in the muscles 

 of fish ; an alimentary canal is said to be present, and there are no 

 roots. 



In the Ascothoracica, all of which are parasitic in Zoantharia or 

 in echinoderms, the mantle may have a bivalved form (Synagoga 

 and Petrarca), or it may form a capacious sac (Laura) much larger 

 than the body with which it is connected by a narrow neck and hav- 

 ing only a small opening to the exterior. In Dendrogastcr the mantle 



