220 ANATOMY OF SPECIAL FORMS. 



Hydractinia eciiinata. 

 Plate XV aiul Plate XVI, figs. 10 and 11. 



In many respects this hydroid departs widely from the usual condition of the order. It is 

 full of interest, whether we regard the very exceptional structvn-c of the hydrophyton or the great 

 extent to which heteromorphism has been carried in the zooids.^ 



The species is found spreading over the surface of dead univalve shells, almost always such 

 as are inhabited by a hermit crab. 



The Ilijdrophyion.- — There is nothing in the morphology of Hydractinia more deserving of 

 attention than the coenosarc with its chitinous excretion. Investing the shell of which the hydroid 

 had taken possession, and usually extending for some distance beyond the shell-mouth, is a 

 continuous layer (PI. XV, figs. 1, 2), soft and fleshy in the living animal, but replaced after the 

 death of the hydroid and the disappearance of its soft parts by a firm chitinous crust, thickly 

 covered by blunt spines. This crust was the only part of the animal with which tlie earlier 

 observers were acquainted, and, being very different from all other hydroid forms then known, its 

 real nature was entirely misunderstood, and it was sometimes regarded as a polyzoon, sometimes 

 as a sponge. It is the chitinous secretion of the fleshy ccenosarc which forms a common basis 

 for the various zooids of the polymorphous colony. 



When a section of this common horizontal basis of the colony is made in the living animal 

 from the more superficial towards the deeper parts, the knife lays open a multitude of polyhedral 

 spaces (PI. XVI, fig. 10). These are the cross sections of freely anastomosing tubes, which, being 

 placed in different planes, form a canaliculated or sponge-like structure. These tubes consist of 

 the soft fleshy ccenosarc, bounded by a wall of chitine, and having their cavity surrounded by a 

 layer of minute cells filled with coloured granules. The fleshy mass itself I believe to consist 

 mainly of an endoderm, the cells with coloured granules which surround the cavity of the tulles 

 corresponding to a similar layer which surrounds the somatic cavity in hydroids generally. The 

 peripheral portion of this endodermal mass appears to be surrounded by a true but very thin ecto- 

 derm, on which devolves the function of secreting the chitinous investment of the canals. This 

 ectodermal layer is sufficiently obvious in the very young coenosarc, and in the superficial canals 

 of the older, but in other portions of the adult ccenosarc it is very difficult to demonstrate it. 



At the free surface of the coenosarcal expansion (PL XVI, fig. 10, a a) its intercommunicating 

 canals (i) are only partially invested by chitine, this excretion being in the superficial layer of canals 

 confined to their deeper parts, thus forming open channels, in which the canals are lodged, so that 

 when the soft parts are removed the chitinous perisarc forms on the surface a nuiltitude of inter- 

 secting ridges having between them the channels which had contained the superficial coenosarcal 

 canals. Upon the whole of the free surface, however, the ectoderm of these canals forms a con- 

 tinuous and very conspicuous layer [a a), having acquired increased thickness and developed in its 



' Dr. Strctbill Wright has given au excellent account of this hydroid. He was tlie first to call 

 attention to the occurrence in it of the spiral zooids. See ' Proc. Roy. Pliys. Soc. Edin.' 



