HABITS OF IIOLOTHURIANS. 



127 



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, and it seems more natural to re- 

 gard the Echinoderms 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 

 Holotlmria, one a northern the 

 other a subtropical and tropical 

 form, and of a Synapta, found 

 everywhere along our coast in sand 

 below tide-marks, will give the 

 groundwork ; and this knowledge, 

 autoptically acquired, can then be 

 corrected and extended by reading 

 ^nonographs or compiled state- 

 ments to be found in the more 

 authoritative general works on 

 comparative anatomy. PIg> 88 ._ Pm , ac , a fl0 ndosa.- 



Living Holothurians can be pro- From Teune y'<* zoology. 

 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. 88). It lives from ex- 



