410 Mr Menon, Notes on Semper s Larvae. 



mesenteries, on account of the presence of these cells, form most 

 conspicuous structures in transverse sections. The cells of the 

 mesenterial filaments agree with those of the lining of the stomo- 

 daeum. The filaments consist of radially arranged, elongated 

 cells and have a rounded outline. 



The youngest stage obtained of Zoanthella was a sterrogastrula 

 of about 1 mm. in length. It had a vermiform body with rounded 

 ends, and was slightly bent towards the ventral side. In addition 

 to the covering of small cilia there was the characteristic longitu- 

 dinal band of long cilia. It had a yellowish colour with irregular 

 dark brown patches (fig. 4). 



In this stage there is a circular mouth placed at one end of 

 the body. A section of this stage is shown in fig. 5. The ectoderm 

 is a well-defined layer, and consists of radially arranged columnar 

 cells. Besides the ordinary cells, cells with granular contents and 

 nematocysts are also present. The nuclei which are round or 

 elongated are arranged in the outer and middle parts of the. ecto- 

 derm. Below this nucleated layer there is a zone which remains 

 uncoloured in stained sections. On the ventral side of the trans- 

 verse section there is a shallow groove which lodges the long cilia. 

 At the bottom of the groove the nuclei are round and lie close 

 together. 



The ectoderm is separated from the inner mass by a thin, 

 structureless lamella. This lamella is very thin throughout and 

 in the region of the ciliated band appears to be almost absent. 

 Within this mesogloea there is a syncytium which completely 

 up the interior of the gastrula. The syncytium contains 

 aumerous scattered nuclei, yolk spherules and large and small 

 vacuoles, some of which are in the living larva filled with fat 

 globules. The vacuoles become larger towards the mouth. Imme- 

 diately below the mouth there is a shallow quadrangular space 

 into which the mouth opens. The space is lined by a continua- 

 tion of the ectoderm, and is thus a short stomodaeum. The colour 

 of the larva seems to be due to the coloured globules in the inner 

 mass. Whether this syncytial mass has been developed secondarily 

 from the simple wall of a coeloblastula as in Manicina, or, as seems 

 more probable, is the inner portion of a solid morula the outer 

 cells of which became differentiated to form the ectoderm, cannot 

 of course be decided till the earlier stages are examined. 



The definitive endoderm is formed from the syncytium in a 

 manner which has been described for Manicina, Geriactis, and 

 other genera. This is well seen in an older larva of which a 

 section is represented in fig. 6. The ectoderm does not differ 

 from that of the preceding stage. The mesogloea remains very 

 thin. Neither here nor in the earlier stage is there any folding 

 of the mesogloea to indicate the commencement of a mesentery. 



