On the Anatomy of the Lametlibranchiata, [65 



extremity of the animal. The larva at the same time becomes 

 lengthened and vermiform ; the girding ciliated bands resolve them- 

 selves into a single transverse band, encircling the body near the 

 anterior extremity, and a band passing below the mouth and longi- 

 tudinally down either side to the tail. 



Large lobulated masses of fine granular tissue occupy the cavity 

 of the body on either side of the alimentary canal. 



The echinoderm-zooid originates, apparently, beneath the integu- 

 ment of the larva, but perhaps in an inversion of that integument, 

 in the form of a rosette of cells encysted near the upper extremity 

 of the intestine. The rosette is at first single, but shortly takes the • 

 appearance of a double ring, the rings being united by a curved tube. 

 These rings seem to represent the rudiments of the ambulacral 

 vascular system of the echinoderm, and the curved tube the origin of 

 the alimentary canal. A dense coating of granular areolar tissue is 

 formed round the young crinoid, obscuring the further development 

 of the internal organs. The mode of its disengagement from the 

 larva was not observed. 



Free from the locomotive larva, the echinoderm in its earliest stage 

 is a motionless, white, egg-like body, covered externally with a thick 

 transparent layer, which is traversed vertically by scattered fusiform 

 oil-cells. 



Beneath this layer are seen rapidly-forming patches of the calcified 

 areolar tissue so characteristic of the class. The body becomes 

 club-shaped ; the narrow end attaches itself by cement-matter to some 

 foreign substance, and a head and stem are distinguished. 



Two corresponding rows of five plates each (the basalia, and the 

 first row of the interradialia) form a calcareous chalice round the 

 base of the head. Rudimentary arms now first make their appear- 

 ance, and the development of the attached pentacrinal form proceeds 

 steadily. 



From his observations of several broods during the spring of 1858, 

 the author was led to believe that, under circumstances favourable 

 to the production of the pentacrinal stage, the development of the 

 larva may be arrested in any of its earlier stages, and before the 

 complete differentiation of its internal organs. It is hoped that the 

 observations of another season may solve this and other questions 

 which still remain somewhat obscure. 



February 3, 1859. — Sir Benjamin C. Brodie, Bart., President, in 



the Chair. 



" On the Aquiferous and Oviductal Systems in the LamelHbran- 

 chiate Mollusks/' By George Rolleston, M.D., Lee's Reader in 

 Anatomy, and Charles Robertson, Esq., Curator of the Museum, 

 Christ Church, Oxford. 



In this paper the authors bring forward two views as to the ana- 

 tomy of the Lamellibranchiata. 



1 . The first part of the communication is devoted to an examina- 

 tion of the commonly -received opinion as to the outlet of the ovarian 



Ann, §• Mag, N. Hist. Ser. 3. VoL iv. 5 



