HABITS OF HOLOTHURIAN8. 



209 



semblance between the two branches (Echinoderms and 

 worms) is one simply of analogy, and involves no blood-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, but it seems more natural to re- 

 gard the Echinoderms as forming a branch of animals stand- 

 ing near such worms, possibly the Nemerteans, as have a body- 

 cavity, as well as a complicated excretory (nephridial) 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 

 Holothuria, one a northern the 

 other a subtropical and tropical 

 form, and of a Synapta, found 

 everywhere along our coast in sand 

 below tide-m&rks, 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 

 comparative anatomy. Fig.i5i.-pterfa 



Living Holothurians can be pro- 

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

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

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

 action of their branchiae 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. 151). It lives from ex- 



