190 



INVERTEBKATE ANIMALS. 



united together by a well-developed nervous system, and are thus 

 brought into organic connection with one another. 



The vast majority of the I'dliiznn are fixed, and thus assume a 

 very plant-like ap[)earauce. There is one fresh-water type, how- 

 ever (viz., i'ristatelld), in which the colony can creep about upon a 

 flattened base very like the foot of a slug. In this same form, also, 

 alone of all the Polyzoa, there is not any outer covering or ectocyst 

 to the polypides. 



The Poli/zoii are partly inhabitants of the sea and partly of fresh 

 water, and they are thus divided into two groups which, as a 



Fig. 1,33. ~1. Fragment of Flu.^tra trnncata, one of tlie Se.a-ni.its, natural size. 2, A 

 single polypide of Valkeria, niagnilied, slinwing tlie circular crown of tentacles, 3. 

 A jiulyinde of Lophopiis criiskilliniis, a fresh-water Polyzodn, liighly magnified, 

 showing the horse-slioe-shaped crown of tentacles ; a Tentacular crown ; h Gullet; 

 c Stomach ; d Intestine ; e Anus ; tj Gizzard ; Ic Endocyst ; I Ectocyst. 



general rule, differ from one another considerably in anatomical 

 structure. In most of the fresh-w.iter I'lj/i/zmi the tentacles are 

 boiLie upon a cresceutir disc or stage (fig. 13.3, 3), so that the crown 

 of tentacles as.sumes the shape of a hor.st-shoe. In almo.st all the 

 marine forms, on the other hand, the tentacles (fig, 133, 2) are 

 simply arranged in a circle. 



All the Poli/zoa are hermaphrodite, each polypide being furnished 

 with the rejiroductive organs proper to the two sexes. The eggs 

 are simply liberated into the perivisceral cavity, where they are 



