Mr Menon, Notes on Semper's Larvae. 409 



The macromesenteries have well-developed mesenterial fila- 

 ments. Gland cells and occasionally the larger kind of nematocysts 

 are seen in them. 



The deeper layer of the ectoderm is in places closely packed 

 with the larger kind of nematocysts. It is very rarely that a 

 whole nematocyst, with capsule and thread, is seen. What one 

 generally finds is a number of threads loosely and irregularly 

 coiled, lying in large oval spaces in the deeper parts of the ecto- 

 derm ; their capsule is visible. These threads are very conspicuous 

 structures in transverse sections. The smaller nematocysts are 

 very numerous in the superficial layer of the ectoderm. The 

 colour of the larva is due to the numerous pigment cells in the 

 ectoderm. The mesogloea is a homogeneous membrane, thicker 

 in some places than in others. It contains cell-elements of two 

 kinds, small, clear, ovoid cells and large, round, granular cells. Of 

 these the former seem to be derived from the similar cells which 

 lie here and there in the ectoderm, immediately next to the 

 mesogloea, just in the same way as the large, granular cells are 

 derived, as is indeed stated by Van Beneden, from the deeply 

 staining cells which lie here and there on the inner face of the 

 mesogloea. The cells of the mesogloea lie in spaces which are 

 often seen without the cells. They do not form strands, as in 

 Zoanthina. 



The endoderm is a layer of great thickness. It is thin in the 

 region of the stomodaeum, but increases in thickness aborally. In 

 sections passing a little below the lower end of the stomodaeum 

 the endoderm forms large masses almost filling up the mesenteric 

 chambers, with their rounded inner ends projecting into the ; 

 coelenteron. The endoderm covering the faces of the mesenteries 

 forms a comparatively thin layer. About a quarter of the animal's 

 length from the aboral end the endoderm almost fills up the 

 coelenteric cavity, a small irregular space in it representing the 

 much reduced coelenteron. Still lower down the cavity disap- 

 pears altogether. The endoderm is highly vacuolated and con- 

 tains numerous scattered nuclei. This condition of the endoderm 

 is interesting inasmuch as it shows that the explanation sug- 

 gested by G. C. Bourne of a similar condition of the endoderm in 

 Euphyllia, viz., that the great development of the vacuolated 

 endoderm filling up the coelenteron is connected with the pre- 

 sence of a large number of Zooxanthellae, is not applicable to all 

 such instances. The endoderm covering the dorsal and ventral 

 faces of the macromesenteries as well as the whole of the endo- 

 derm covering the micromesenteries, consists of large granular 

 cells or masses of cells with one or more nuclei. It stains very 

 deeply. Similar deeply-staining cells often form a core to the 

 large vacuolated masses projecting into the coelenteron. The 



