HABITS OF HOLOTHURIANS, 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-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 
comparative anatomy. Fig.151.—Pentacta Srondosa,— 
Living Holothurians can be pro- From Teuney’s 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 branchiz 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. 
