HYDROIPS. 77 



outer shell breaking away, the embryo, still with a delicate shell around it, escapes into 

 the water ; a cleft appears in the body-wall, which becomes the mouth ; the tentacles 

 are developed, and the embryo bursting its thin shell, appears as a young Hydra. The 

 development of Hydra is thus seen to be simple and continuous ; there are no great 

 or sudden changes such as occur in the life-histories of so many other animals. 



There are a number of so-called species of Hydra found in the United States, the 

 most common of which are a green one known as Hydra viridis, and a light-brown one 

 called Hydra fusca. The latter often attains a much larger size than the former, and 

 on account of its being much more translucent, is a better kind for study. They are 

 found in slow or stagnant water, and are sometimes so very abundant as to form a 

 delicate, fringe-like covering over every submerged object, in quite a large pool. 

 Hydra has also been found once in a brackish arm of the sea in Germany, by Marshall. 

 Having obtained a general idea of one hydroid, we may now take up the systematic 

 arrangement of the group, considering the various sub-orders and a few of the most 

 prominent families. 



Sub-Oedbe I. — Eleutheeoblastea. 



This, the lowest sub-order, has for its type the genus Hydra, which has already been 

 described at length. No other genus belonging to this group is known. This sub- 

 order is destitute of a hardened body-envelope, and the zooids of the body, or troph- 

 osome, are never firmly attached. Even more simple than Hydra is the peculiar genus 

 Protohydra found by Greef in the ocean at Ostend, Belgium. It can be best described 

 by saying that it closely resembles Hydra, except that it entirely lacks the tentacles so 

 prominent in that form. It reproduces by transverse fission. So little is known of 

 the structure and growth of Protohydra that the position which it is made to occupy 

 in our classification must be regarded as provisional. 



Sijb-Oedee II. — GymjStoblastba. 



All the members of this division have a hardened body-envelope called the perisarc, 

 and live in colonies which are always attached to some foreign support. From the 

 next division of the same rank, they are separated by never having the reproductive 

 and nutritive portions enclosed in a chitinous capsule, and the generative zooids do not 

 usually become free, independently developing organisms. The generative zooid, escaped 

 from its parent, may have a medusa form, from which ultimately a large number of ova 

 are dropped, or it may assume the condition called the actinula, an oval body floating 

 passively about or creeping on the bottom. In those hydroids which have an actinula 

 this body develops directly, without intermediate metamorphosis, into a hydroid of 

 the same form as that from which it sprung. In some of the gymnoblastic hydroids 

 there are no free medusae and no actinulse, properly so called, but a locomotive zooid, 

 called a sporosac, which performs the same function. The sporosac is a ciliated body, 

 capable of active locomotion, and possessed of two tentacles. It carries in its cavity 

 a single ovum. In many of the young gymnoblastic hydroids, the embryo leaves the 

 mother's care as a planula, which develops directly into a hydi-oid similar to that from 

 which it originated. 



With this sub-order a new feature is introduced. In Hydra we found the nutritive 

 and reproductive systems united in the same individual, but here we find certain por- 



