68 MORPHOLOGY. 



a. Development of the HydranUi. 



It is exceedingly rare to find the trophosome retaining through life the simple condition 

 which it presents during its primordial state. Cases, however, of permanently simple tropho- 

 somes occur. We meet with them, for example, in Corpnorpka (PI. XIX) and certain aUied 

 forms. The curious free trophosome of Nemopsis as described by M'Crady, and of Acaulis, as 

 described by Stimpson, are probably only the detached hydranths of some fixed Tubularians 

 which may possess the habit of throwing off" their hydranths, as we know to be the case in 

 certain European species of Tubularia. 



Ihjdranili-hud in the Gymnohlastea and Eleiitlieroblastea. — When a hydranth-bud is about to 

 become developed from any part of the coenosarc in the gymnoblastic hydroids, the two layers of 

 the coenosarc are seen at this spot to be pushed outwards as if by an incipient hernia, and the 

 little hollow tubercle thus produced forces before it the investing perisarc, which is first extended 

 over the advancing bud, and — except in the very young parts, where it is still in the condition of 

 a mere film — is at last absorbed or ruptured. 



The little bud, however, has been in the mean time clothing itself with a new perisarc, 

 which, now that it has escaped from the confinement of the old one, is seen to cover it with a very 

 delicate, transparent, structureless pellicle. The bud continues to increase in size, becoming 

 longer and thicker, with its endoderm and ectoderm very distinct, and with its cavity opening 

 freely into that of the branch from which it springs, and admitting into its interior the fluid with 

 the floating granules which fill the general cavity of the coenosarc, and which are kept in a state 

 of active rotation within the bud. It continues to enlarge, but has its distal extremity still 

 closed, while the entire bud is still invested by its delicate perisarc (PI. II, fig. 5, «fec.). 



We next find that the little bud has acquired a somewhat clavate form by the enlargement 

 of its distal extremity. While the perisarc which clothes the growing bud continues, by means 

 of new layers deposited upon its inner surface, to increase in thickness over the proximal part 

 of the bud, these new layers cease, in almost every case, at a very early period to be excreted from 

 the free extremity of the bud, and the perisarc here accordingly remains in the condition of a 

 transparent structureless pellicle of extreme tenuity, which at last, in most cases, entirely dis- 

 appears. We now find tentacles begin to grow out from the enlarged extremity of the bud, and 

 a terminal mouth to become developed ; the form is thus gradually assumed which is to charac- 

 terise the adult hydranth. 



In some cases, however {Coryne voginata, Hincks (PI. IV, fig. 8), and Eudendrium vat/inatum, 

 Allm. (PL XIV, fig. 7) ), the perisarc which clothes the free extremity of the growing branch 

 attains considerable thickness, and does not disappear until a later period ; but it ceases in such 

 Gases to be in close contact with the ectoderm, and forms an outer chitinous capsule, within 

 which the hydranth continues to become developed ; and this development proceeds to the 

 formation of tentacles and the assumption, more or less, of the adult form of the hydranth-bud, 

 before the rupture of the enclosing capsule places the young hydranth in direct relation with the 

 surrounding water. 



