58 N. Annandalf : The Fauna of Brackish Ponds. [VOL. 1, 



the filament, on the other hand, corresponded with a region of 

 the mesentery containing, at that date, cells with all the char- 

 acters of sexual cells but as yet of an indeterminate nature. 

 These cells were situated at the base of the endoderm covering the 

 mesenter}-. The ova were already far advanced in the part of the 

 mesenter)^ occupied by the ovary, and this part of the mesentery 

 had lost its purple colour ; but the lower part, below the ovary, was 

 still of a very deep purple. The structure of the ovary closely re- 

 sembled (except that the whole structure was strongl}^ folded) 

 that of the ovary of Calliactis parasitica as figured by O. and R. 

 Hertwig (3). In specimens of the new race of M. schillerianum 

 killed in January, however, the condition of the gonads had altered 

 completely. The upper part of the mesenter}^ was now devoid of 

 ova and was thin and colourless ; the lower part, in which the 

 indeterminate sexual cells had occurred in other individuals a month 

 earlier, was now distended with spermatozoa arranged in approxi- 

 mately quadrangular follicules. Although they were already ripe, 

 the development of the testes had not destroyed the purple colour 

 of this part of the mesentery. The structure of the organs was 

 identical (except for a folding similar to that of the ovaries but 

 even more marked) with that of the testes of Calliactis parasitica. 

 In a few individual mesenteries the testes appeared to have in- 

 vaded that region which had been previously occupied by the ova- 

 ries, but the two regions were as a rule distinct, and corresponded 

 to those parts of the mesenterial filaments which I have referred to 

 above. In individuals killed towards the end of March the gonads 

 were again in the same condition as in those killed in December. 



Stoliczka states that the eggs have a chitinous covering when 

 emitted, and that there is a dark layer beneath this covering. If 

 his statements are correct, both these structures must come into 

 existence at a very late stage of development, for ova which appear 

 to be of nearly full size show no trace of either. The spermatozoa, 

 as Stoliczka noted, have a round head and a tail of somewhat mod- 

 erate dimensions. In the testes they are arranged with their heads 

 pointing outwards towards the endoderm which encloses them, and 

 it appears that the movements of their tails prove sufficient to drive 

 them through this endoderm , probably between the cells. Stoliczka's 

 specimen, which threw out part of the gonad, was evidently living 

 under unfavourable conditions, and the process appears to have been 

 pathological. In individuals of the form he described living in my 

 aquarium the gonads degenerated altogether. These individuals 

 were obtained, together with others which were killed and dissected, 

 in the Matla estuary at the beginning of January. The gonads of 

 those which were examined were, at that date, in exactly the same 

 condition as examples of the isolated race from the ponds. 



Skeleton. — 



In his account of the species Stoliczka stated that it was 

 remarkable in the possession of a skeleton consisting of both cal- 

 careous and silicious elements. I have examined both his own 



