TUBULARIA INDIVISA. 205 



ANATOMY OF SPECIAL FORMS. 



The following are some special studies of hydroid anatomy which I have deemed it right 

 to bring together in this place, in order to render more complete the general exposition of the 

 Hydroida attempted in the present work. They have been selected so as to afford examples 

 of the more important morphological variations met with among the gymnoblastic forms — such, 

 at least, as I have had an opportunity of studying. Each includes the anatomy of a species 

 which may be taken as the type of some generic group. 



TuBULARIA INDIVISA. 



Plates XX and XXIII. 



This beautiful and by no means uncommon hydroid, one of the largest and most interesting 

 of the entire order, presents clusters of unbranched stems which spring from a creeping, tubular, 

 branching and anastomosing stolon, attaining a height of many inches and a thickness of about a 

 tenth of an inch, and carrying each a hydranth on its summit. 



Ilie Ihjdrophjton. — The entire hydrophyton is invested by a firm chitinous perisarc, in 

 the older parts of which layers of deposition may be detected. The coenosarc is closely embraced 

 by the perisarc except in that part of the stem which lies immediately below the hydranth, 

 thus differing from the majority of the Hydroida, in which the coenosarc for the greater part of 

 its course is separated by a considerable interval from the surrounding perisarc, with which its 

 connection is maintained only by irregular transverse processes of the ectoderm, which here and 

 there stretch across the interval from the one to the other. In the present species, however, as 

 well as in the Tubularice generally, it is only for a short distance below the hydranth that the 

 coenosarc has withdrawn itself from contact with the surrounding perisarc which here forms a 

 thin membranous sheath with large irregular annulations (PI. XX, fig. 2). 



By means of a carefully made transverse section of the stem the structure of the coenosarc 

 may be rendered apparent. If the section be made in any part of the course of the stem excpet 

 immediately below the hydranth, a very remarkable stnicture will be brought into view 

 (PI. XXIII, fig. 7). The endoderui will be here seen to consist of two distinct portions, a peri- 

 pheral and an axial. The axial portion (d) consists of large cells with clear colourless contents, 

 and occupies as if with a sort of pith the entire axis of the section, which is cpiite destitute of an 

 axial somatic cavity. The peripheral portion (c) is composed of small spherical cells filled with 

 coloured granules. In this portion numerous empty spaces may be seen ; they are somewhat 



