TRANSACTIONS OF SECTION D. 647 



opportunity of investigating this point, and in specimens in his possession I have 

 along with him seen the element in question distinct in Phoca barbata, P. green- 

 landica, P. vitulina, P. hispida, and Halichmrus grypus ; also in Otaria jubata 

 and Arctocephalus ursinus. In a young walrus I can make out the exact extent 

 of the corresponding portion of hone, hut in my specimen no indication is ati'orded 

 of separate ossification. I attach great importance to this osseous element in the 

 seals, as it adds a very important argument in addition to the other evidences that 

 the paroccipital of the tish is a true occipital element, and that the mastoid, 

 recognised hy Cuvier and Owen in the fish is homologous with the mastoid of the 

 mammal, and that in these matters Huxley was in error. 



7, On the Habits of the Predaceous Flies of the Family of Asilidce. 

 By Professor E. B. Poulton, F.B.S. 



8. The Development of the Common Sea-urchin (Echinus esculentus). 

 By Professor E. W. MacBride, D.Sc. 



The paper concerned the results of an investigation which had heen carried on 

 for the last four years. The difficulty of rearing the larvae was first pointed out, 

 and the fact that much work had heen done on these larvae by zoologists who 

 were apparently ignorant of the conditions necessary for their healthy existence. 

 Each larva when it had attained the size of a pin's head required lor its healthy 

 existence at least a pint of water changed frequently. The precautions necessary 

 for the successful rearing of these larvae would prohahly lead to good results if 

 applied to cultures of larvae helonging to lohsters and other animals of economic 

 importance. 



The primary body-cavity, or blastocoele, was shown to contain a thick solution 

 of proteid matter, which became thinned as development proceeded ; and as this 

 development consisted largely of processes of infolding of the wall of the spherical 

 vesicle of which the larva at first consisted, the thinning of the solution might 

 bring about a reduction in pressure which rendered the infolding possible. 



The secondary body-cavity exhibited a division into three metameres on each 

 side, and there was a larval brain at the extreme front apex, both of which features 

 recalled strongly the Tornaria larva of Balanoglossus. 



The development consisted of an enlargement of the middle di^nsion of the 

 secondary body. cavity on the left side and of the floor of an ectodermic pit which 

 had come in contact with it, from which the nerve-ring of the adult was formed. 

 This floor became covered with a false floor, due to the union of five interradial 

 ridges, from which the ventral surface of the adult was formed. Aristotle's 

 lantern, the masticatory apparatus, was formed from five pockets of the most 

 posterior division of the ccelom, and was homologous with the perihaemal ring of 

 asterids. 



The young sea-urchin after metamorphosis went through a stage which might 

 be described as the asterid stage. The blood system was a remnant of the primary 

 body-cavity adjacent to the gut and infiltrated with the products of digestion. 



9. The Relationships of the Larva to the Adult in the Starfish, 

 By Dr. A. T. Masterman. 



10, On the Cause of Salmon Disease. By J. IJume Patterson.^ 



The previous investigations on salmon disease have gone to prove that it is 

 caused by the fungus Saprolegnia ferax. Most of the investigations, however, 



' Published in esften$o as a Parliamentary Blue Book by the Fishery Board for 

 Scotland, 



