HABITS OF UOLOTHUBIANS. 



127 



semblance between the two brandies (Bchinoderms and 

 worms) is one simply of analogy, and involves noblood-rela- 

 -tionship. On the other hand the radiated arrangement of 

 parts and the development and relations of the water-vas- 

 ■cular system ally them, through the Ctenophores, with the 

 Actinozoa and Hydroida, and it seems more natural to re- 

 gard the Bchinoderms as forming a branch of animals in- 

 termediate between the Hydroida and the worms, there 

 being certain low worms with a water-vascular system. 



But the student will be better 

 able to appreciate these general 

 •questions after a more or less 

 thorough acquaintance with the 

 forms and structure of the pres- 

 ent group. For this purpose he 

 -should first examine living sea- 

 cucumbers, and then carefully 

 dissect them. A detailed study 

 ■of the anatomy of a Pentacta or a 

 Holotliuria, one a northern the 

 other a subtropical and tropical 

 form, and of a Synapta, found 

 -everywhere along our coast in sand 

 TdcIow tide-marks, will give the 

 groundwork ; and this knowledge, 

 autoptically acquired, can then be 

 -corrected and extended by reading 

 monographs or compiled state- 

 ments to be found in the more 

 authoritative general works on 

 comj)arative anatomy. 



Living Holothurians can be pro- 

 cured with the dredge or dug out of the sand between tide- 

 jmarks. They should be kept in aquaria, and their move- 

 ments watched as well as their mode of locomotion, and the 

 action of their branchias or external gills (tentacles). 



The common sea-cucumber, north of Cape Cod, and ex- 

 tending through the Arctic regions around to Great Britain, 

 is Pentacta frondosa Jaeger (Fig. 88). It lives from ex- 



_, W.— Pentacta fiondosa.— 

 From Teuney's Zoology. 



