HYDROIDA 



On account of their dependence on outward conditions and their power of plastic accommodation 

 to biological influences, the gonophores are unsuitable for basis of division into genera. On the other 

 hand, the genera, owing to the conditions of the gonophores, often fall into a series of biological 

 groups (or stibgenera). The systematists who lay the main stress on the gonophores in establishing 

 the system, in fact apply biological conditions as fundainentum divisionis, and let phylogeny, the pro- 

 perly governing principle of systematic inquiry, recede into the shade. 



b. The comparative anatomy of the nourishing individuals, and the system of the 



athecate hydroids. 



An exact review of the researches of the last years makes ever clearer, as also appears from 

 the statements above, the correctness of Lev ins en's view (1893), maintaining that in the great classifica- 

 tion of the hydroids the main stress must be laid on the peculiar conditions of the nourishing polyps, 

 and reducing at the same time the modifications of growth and the conditions of the gonophores as 

 characters of subordinate importance. Later investigators ' have, indeed, attached ever more importance 

 to the conditions of the polyps. But in so doing they have almost exclusively taken into considera- 

 tion such morphologic criteria as urge themselves on a superficial view of the polyps. The inner 

 anatomy, on the contrary, has been disregarded. Kiihn (1913) certainly by the way points out that 

 the inner anatomy can be different in the different groups. He treats (1. c. p. 50) at some length 

 of the peculiar structure of the polyps of the Tubulariidce, and points out the multifarious development 

 of the thecaphores. But in drawing the bases of his system he makes no attempt to turn these features 

 to further account 



As a result of searching inquiries, the structure of the polyps in the different genera and 

 families has turned out not to be quite so homogeneous as it has been generally held. Both in the 

 construction of the ectoderm and in that of the endoderm differentiations occur, which may be char- 

 acteristic of greater or smaller groups of species and give us several holds for judging the systems 

 drawn up for hydroids in the course of time. 



Therefore, it will here be appropriate to give a brief synopsis of the more important peculiarities 

 of anatomy distinguishing the various groups of athecate hydroids, in order to apply them afterwards 

 to drawing up the system of the group. 



The ectoderm, deciding by the disposition of its elements whether the tentacles have to be 

 claviform or not, has, to some extent, been turned to account as fundamentum divisionis. The clavi- 

 form shape is particularly due to the accumulation of the stinging cells on the tips of the tentacles, 

 while the tentacles are filiform when the nematocysts are more equably distributed. A third type of 

 tentacles, which, as far as is known, is found with all thecaphore hydroids, occurs in the Eudendrhda; 



i Works like that of Poche (1914) I leave out of account That sort of "zoology" which is based not on study of 

 the organisms themselves, but only on what may be beaten up from books, here debouches in the construction of airy castles 

 of complicated systematics, which does not advance zoological science by a hairbreadth, but only contributes to increasing 

 the systematic confusion. Between "regnum" and "family" are inserted 34 thirty four degrees. It is a matter of regret 

 that we do not learn how many osculant categories must be placed between "family" and "individual" to give a "full" picture 

 of nature, lint this will suffice to illustrate the scientific value of the work. 



