HicKsoN — Notes on a Small Collection of Hydrocovallince. 505 



I have only to add a few words concerning the histology of the 

 gastrozooids and dactylozooids. In PL xix. fig. 2, 1 have drawn an 

 ideal longitudinal section through a gastrozooid {gz.) and a pair of 

 dactylozooids [dz.]. It is, of course, impossible to obtain a section 

 in practice similar to that I have drawn in consequence of the 

 curvature of the pores. 



The gastrozooid is provided with a large mouth {M.) and four 

 small tentacles {T.). The tentacles are frequently very difficult 

 to observe both in sections and whole specimens, as they become 

 tightly adpressed to the body wall ; but I have no doubt that 

 Moseley was right in making the general statement that there are 

 four tentacles to the gastrozooids. The ectoderm is thin, finely 

 granular in appearance, and provided with a row of nuclei. I 

 cannot find in my sections any very well marked cell outlines. 

 At the oral extremity of the gastrozooid the endoderm bears a 

 number of clear pyramidal gland-cells, wedged in between the 

 ordinary columnar endoderm-cells. The long lancet-like style 

 stands up in the enteric cavity and is covered by a thin sheath of 

 flattened endoderm-cells. 



The dactylozooids have a solid endoderm, with the cells and 

 nuclei arranged in the same manner as is usual with the solid 

 dactylozooids of other Stylasterids and the tentacles of many 

 Hydrozoa, giving an appearance like a ladder with a nucleus at the 

 median point of each rung. An examination with a high power 

 shows that the histology is not so simple as it appears at first 

 (PL XIX. fig. 3). Although the general arrangement of the slips 

 of granular protoplasm containing the nuclei is at right angles to the 

 long axis of the dactylozooids, and parallel with one another, yet 

 many of them are branched at their extremities or joined with their 

 neighbours by median communications. The clear spaces between 

 the protoplasmic slips are to be considered intracellular, not inter- 

 cellular, in position. In fact we have here a nucleated syncytium 

 with large vacuoles containing probably water only, the stiffness 

 of the dactylozooid being produced by turgidity, as in many vege- 

 table tissues. At the base the dactylozooid passes into a muscular 

 slip attached to the side wall of the pore, the scalariform endoderm 

 becomes irregularly parenchymatous, and between it and the 

 ectoderm there is a thick sheath of nucleated muscle-cells. How 

 far the muscle-cells extend towards the free end of the zooid it is 



