ON THE LABORATORY OF THE MARINE BIOLOGICAL ASSOCIATION. 469 



Inrestifjattons made at the Laboralori/ of the Marine Biological Associa- 

 tion at Pli/mouth. — Report of the Committee, consisting of Mv. G. G. 

 Bourne (Chairman), Professor E. Ray Lankester (Secretari/)^ 

 l^rofessor M. Foster, and Professor S. H. Vines. 



I. Oil <i Blood-fnrminri Orrjan in the Larra of Magelona. Uij FLORENCE 



J5i:CHANA>', ii.Sc 4(;<> 



II. On the Arrvoun Si/xteiu of the Emhrijomc Lobster. Bij Edgae J. 



Allen, B.Sc 4 70 



]ir. Onthv Eohinoderm Fauna of rUjiiiiiuth. i/y J. C. Sl'mner . . .471 



The Committee were appointed to enable Mr. Edgar J. Allen or other 

 zoologist to investigate the Decapod Crustacea, and Mr. J. J. Lister to^ 

 work at the Foraminifera at the Laboratory of the Marine Biological 

 Association. 



The Committee have received a report from Miss Florence Buchanan, 

 B.Sc, hold over from last ye^r on account of ill-health, in addition to Mr. 

 Edgar J. Allen's report, and that of Mr. J. C. Sumner, who occupied the 

 taljle in January and February 189o. 



I. On a Blood-forming Organ in the Larra of Magelona. 

 By Florence Buchanan, B.Sc. 



In August of 189.3 the British Association kindly allowed me the use 

 of their table at Plymouth for the purpose of studying the development of 

 Magelona. I was unfortunately prevented from working out the material 

 collected there in time to present my report to the last meeting ; and even 

 now what new observations I have to record concern the development 

 of the vascular system only, and I must leave the development of the 

 other organs, or systems of organs, to be described later by myself or 

 some other investigator from a more extended series of stages than I at 

 present possess. 



There is a good deal of individual variation with regard to the time 

 of the first appearance of the vascular system, as there is, indeed, 

 with regard to that of other organs also, in the lar\a of Magelona. I 

 have seen a well-developed pulsatile dorsal vesicle, the so-called ' heart,' 

 in larvfe with only eight or nine segments, all bearing provisional chastse, 

 whilst in other larva3 with a good many more segments there has been 

 no trace of such structure, nor of vascular sytem at all. The ' heart '' 

 which is always the first part of the system to make its appearance, and 

 the dorsal and ventral vessels after it are formed, to begin with, by the 

 accumulation of a transparent fluid between tlie splanchnic layer of 

 mesoblast and the hypoblast, beginning anteriorly, and thereby causin"- 

 a pouching of the splanchnic mesoblast in front to form the walls of the 

 'heart,' which lies mainly in front of the hypoblastic portion of the 

 alimentary canal and over the pharynx, and gradually extending back- 

 wards. By the time that the larva has reached the stage in which the 

 body is divided into three regions, and sometimes before the loss of the 

 provisional chietfe of the middle region, there is seen in the livino' larva 

 at the posterior end of the dorsal vessel, or of what is goino- to become 

 the dorsal vessel, and in the middle region of the body, a dark reddish- 

 brown mass. This is seen in sections to consist of a much swollen 

 portion of the splanchnic mesoblast in which there are many nuclei, but 

 no distinct cell boundaries, and completely blocking up the space between 

 it and the hypoblast. In later stages, when the splanchnic mesoblast has 



