2 THE NEMERTBANS. 



who found his Folia tetrophthalma (a Tetrastemma with large eyes) in great abundance in the 

 respiratory cavity of " Ascidia mammellata!' Leuckart and Pagenstecher also state that the 

 former got at Nice a pale Tetrastemma in all the stages of egg, young and perfect animal — living 

 parasitically in the body-cavity of Phallusia mamillaris ; such, however, in all probability, being 

 only a confirmation of the foregoing. A. Agassiz, again, found a species of Planaria, which he 

 thinks identical with the Planaria angulata of Miiller, on the under surface of the base of 

 the tail in Limidus. This habit had also been observed in another Planaria, that frequents 

 Velella in the Atlantic, by Lesson in his zoology of the ' Voyage autour du Monde sur la 

 corvette La Coquille ;' and by Schneider in the case of Anoplodium parasita (one of the Rhab- 

 docoela), which inhabits Holothuria tubulosa. Further observations will in all likelihood lead to 

 the discovery of parasitic species in the Medusae. Such do not seem to be true parasites like the 

 Entozoa, but may appropriately be grouped under the comprehensive title " Commensalisme," 

 recently constituted by Prof, van Beneden — in an interesting lecture delivered before the Royal 

 Belgian Academy. The animals included under this head do not prey upon the juices of their 

 hosts ; but, like the Adamsia attached to the shell containing the Pagurus, or the accompanying 

 JVereilepas, they simply live together for their mutual comfort and convenience. 



Comparatively few specimens, and these generally the largest forms, are to be found in our 

 museums ; and even such examples, if named at all, are often specifically confounded, the same 

 animal, e.g., Lineus marinus, being characterised by many names. In looking over such col- 

 lections, indeed, one meets with a curious nomenclature ; thus I have found a large Sipunculus 

 labelled ' Serpentaria,' a JBipalium ' Meckelia/ an elongated Sgnapta and a Tania respectively 

 termed c Lineus,' and not a few distinguished by the ambiguous title of ' leech/ This confusion 

 is partly due to the great changes that ensue on placing the animals in spirit. Specimens mea- 

 suring feet or even yards in length shrink to short processes a few inches long, and the contour 

 of the head is often indistinguishable on account of its retraction within the anterior portion of the 

 body. Moreover, although the worm is in a manner preserved, it is generally unfit for dissection, 

 and the colours frequently fade. While difficulties thus beset the investigator of specimens in 

 museums, the obstacles to the satisfactory examination of the living forms are scarcely less per- 

 plexing ; and though I would not fully endorse the description of Sir J. Dalyell, yet there is 

 much truth in his observations : — "That many worms have no external prominences rising above 

 the smoothness of their skin, or depressions sinking into it. That neither specks nor eyes, nor 

 the position of the mouth can be discovered in the living specimens; that the student of 

 animated nature cannot destroy his subject, and if perishing in his possession, it often goes so 

 speedily to decay, that it is impossible, were he even a skilful anatomist, to avail himself of 

 dissection." 



The colours of many species of the group are of such beauty as to attract even the casual 

 observer, while in this respect also they widely deviate from their supposed allies the parasitic 

 worms. The richest purples appear on velvety skins of deep brown or black, each of the soft 

 and mobile folds giving shades that vary in intensity and lustre. Bright yellow contrasts with 

 dark brown ; white with vermilion, brown and dull pink ; while individual uniformity is charac- 

 terised by such hues as rose-pink, white, green, yellow and olive, the gradations of colour in the 

 various parts of a single specimen being so subtle that enthusiasm as well as skill is necessary in 

 the artist who sets himself to the task of faithful delineation. Our indigenous species as a whole 

 do not seem to be less brilliantly coloured than those of warmer climates, if we may judge from 



