St.- Andrews Marine Laboratory . 149 



As none were procured during the winter months, these were, 

 in all probability, either developed from the Scyphistoma- 

 stage {Hydra tuba of Dal jell), so abundant on the rocky 

 shores, or from suitable sites in deeper water, and carried by 

 tides and their own movements where they were found. As 

 the season advanced a few appeared (e. g. at the beginning 

 of July) on the surface of the sea off the Yorkshire coast ; 

 but this month was well advanced before they came in 

 numbers to the surface of St.-Andrews Bay. Very smooth 

 water and evening seemed to be favourable ; indeed they have 

 been observed swimming about a fathom or more under water 

 by day and then rising to the calm surface as evening- 

 approached. Then their contractions were visible as tremors on 

 the glassy surface at a considerable distance. Their hues still 

 remained of the same fine purplish blue. In August a 

 general increase in size was noticeable in the crowds of this 

 form, often observed at and near the surface. Many, more- 

 over, exhibited a decided change in colour, being brownish 

 pink and brownish, leading on, in short, to the fine brownish 

 shade of Cyanea capiIlata } Esch., which occasionally came up in 

 the trawl from deep water, or was captured amongst the others 

 near the surface. Some of the larger forms showed the tran- 

 sition clearly, being for the most part, of the chocolate-brown 

 hue with just a trace of the purplish tint here and there. Vast 

 numbers were thrown on the west sands in September, chiefly 

 the developing forms, though some very large adults were 

 amongst them, one, about the beginning ot' October, measuring 

 26 inches across the disk. So far as could be observed, the 

 structure of the two forms above mentioned is identical. The 

 reproductive organs in the purplish forms examined were 

 mostly female, while in the case of the large brown examples 

 males alone occurred. This, however, is probably accidental. 



The mussel (Mytilus edulis, L.) differs from the common 

 oyster in the complete separation of the sexes, each example 

 being either male or female. In the undeveloped condition 

 the structure of the organs seems to be similar in both sexes, 

 while in the mature the shape of the valves gives no reliable 

 distinction. Comparatively small specimens are found with 

 fully developed sexual organs. Thus a male measuring in 

 length I4 inch had spermatozoa as fully formed as a large 

 example, and so in regard to the ova in a female measuring 

 If inch. The reproductive elements are developed in the 

 mantle. 



The mussels were examined from January to July, with the 

 following preliminary results : — Compared with an undeve- 

 loped specimen, the mantle in January is considerably thicker. 



