332 SECTIONAL TRANSACTIONS.— D. 



of muscles, by the contraction of which it is kept in almost continual oscillation in the 

 living larva. 



Graber in 1878 attributed to the organ the function of an otocyst, to which it 

 bears a general similarity. Later investigators argued that it was either a gland or a 

 sound-producing organ, but neither of these latter contentions can be readily accepted. 



In twenty anaesthetised larvae belonging to four distinct terrestrial species of 

 Tabanus, the organ was successfully excised. On recovering from the effects of the 

 anaesthetic the larvae fed regularly, moulted and even pupated, producing in two 

 instances perfect adults. In no case was the organ regenerated at a subsequent 

 ecdysis. After the operation the larvae responded to a variety of stimuli just as 

 readily as they did before the organ was removed, and no lack of co-ordination was 

 discernible. On this evidence Graber's organ is not a mechanism for the regulation 

 of the organism's movements, an attribute commonly associated with otocysts. It 

 may, however, serve to detect disturbances in the surrounding medium, and in this 

 respect would serve a useful purpose both in free-swimming and terrestrial tabanid 

 larvae. 



Friday, September 2. 



Miss S. M. Manton. — On the Embryology of a Mysid Crustacean. 



Only a few points of more general interest can be considered. The germ layers 

 and genital rudiment are differentiated externally on the germinal disk prior to 

 gastrulation. The spatial relations of the germ layers in the Malacostraca are funda- 

 mentally different from the lower Crustacea in both yolky and non-yolky forms. 



The mesoderm is formed in three ways. There are preantennulary and seventh 

 abdominal somites, and coelomic cavities appear in most segments. The walls of the 

 preantennulary coelomic sacs form the stomodoeal musculature and anterior aorta 

 just as in Limulus and Spider. Paired mesodermal bands in the ' naupliar ' segments 

 never contain cavities other than the antennal gland end sac. ■ The whole antennal 

 gland is mesodermal. The trunk mesoderm is formed by a row of eight teloblasts. 

 Paired ccelomic cavities occur in all but the maxillulary segment. A dorsal vessel is 

 developed from the coelomic sacs much as in Estheria ; and a ' cardiac plug ' is formed 

 as in Chirocephalus. The seventh mesodermal somites lie behind the sixth segment 

 which bears the uropods. 



The endoskeletal system is ectodermal, and consists of (1) hollow ectodermal 

 intuckings of the nature of apodemes, and (2) transverse segmental bars whicli separate 

 from the ectoderm forming median tendons of the mandibular adductor and trunk 

 mesodermal muscles. 



Much of the musculature is ectodermal in origin. 



Prof. E. W. MacBride, F.R.S. — On the Heart of the Larva of the Sea-urchin 

 (Echinus miliaris). 



The existence of a pulsating vesicle or ' heart ' in Echinoderm larvae has been 

 asserted ever since the time of Metschnikoff, who described it in the larvae of 

 Sea-urchins in 1864. It was rediscovered and more exhaustively described bv Burv 

 in 1889 and again in 1896. 



A similar vesicle was described by Gemmell in the larvse of starfish in 1912, and 

 subsequently in those of Ophiurids in 1916. 



In the course of experiments made with a view to modifying the normal course 

 of development I have reared the larvae of Echinus miliaris through the entire course 

 of their larval development until the conclusion of metamorphosis every spring for 

 the last fifteen years. 



The larvae of this species are peculiarly transparent, and so are well adapted to 

 show their internal structure when living. 



The pulsating vesicle can be made out in a larva about a fortnight old ; but it 

 is best observed just prior to metamorphosis. It lies above the oesophagus just under 

 the madreporic pore. 



The wall of the vesicle is the ventral wall of the dorsal sac ; the contents of the 

 vesicle are blastoccelic fluid contained between this wall and oesophageal epithelium. 



The ' beating ' is a wriggling forwardly directed peristalsis which must drive the 

 contents forward. It is apparently dependent on a good oxygen supply, for if a 

 larva be confined in a hollowed slide under a coverslip peristalsis occurs only at long 



