60 M. E. Mecznikow on the Rhabdocoela. 



by Claparede*, and found by him on the Scottish coasts, is evi- 

 dently very nearly allied to the animal just described. 



Both these animals were found in a sexless state, and there- 

 fore regarded as larvse. Leuckart f remarks, upon the form de- 

 scribed by Busch, " Alaurina prolifera is certainly a larval 

 worm, although it may be doubtful to what group it belongs." 

 The anatomical structure of the larva in question is very imper- 

 fectly described in the memoirs just cited. It was the more in- 

 teresting to me, therefore, to meet with several specimens of an 

 ianimal nearly allied to Alaurina, which I found, in August last, 

 upon the surface of the sea near Heligoland. 



All the specimens found were composed of four parts (PL VIII. 

 fig. 6), of which the foremost was the longest, whilst the other 

 three were nearly of equal length. The total length of the 

 animal was 1^ millim. The anterior part was furnished with a 

 tactile proboscis, as in the animals of Busch and Claparede, — 

 therefore with an apparatus which may be regarded as a group- 

 character. Its pale-green colour distinguishes it from the rest 

 of the body, which is citron-yellow, and covered with a dense 

 coat of fine cilia, which are entirely wanting on the conical pro- 

 boscis. The stronger vibratile hairs described by Claparede on 

 his larva are not present in my animal; but, on the other hand, 

 it bears a long seta at its posterior end, and this may probably 

 be identical with those oi Alaurina prolifera. 



The cilia are inserted upon the isolable spherical epithelial 

 cells. Under the skin the body is surrounded by a distinct 

 layer of annular muscular fibres. Of the nervous system I have 

 found no trace in my animal ; but there is a pair of small black 

 eye-points behind the proboscis. The eyes do not usually occur 

 upon the three hinder parts of the body ; once only have I seen 

 a pair of such organs upon the last " segment." 



The mouth is situated on the ventral surface, behind the eyes. 

 It leads into a ciliated buccal cavity, which narrows and then 

 leads into a pharynx (fig. 6 ph.) provided with strong muscles 

 (nearly as in the Mesostomece). The intestine runs straight 

 through the whole body ; I could not observe its posterior ori- 

 fice (anus), any more than Claparede, whilst Busch describes 

 his Alaurina as an animal provided with an anus. I believe that 

 these characters in my animal are very similar to those of the 

 MicrostomecE, in which I have likewise sought in vain for an 

 anus. In Microstomum lineare, under a moderate pressure, I 

 always saw the contents of the intestine issuing only from the 

 mouth. Do the Microstomese really possess no anus? and is 



* Recherches, &c. p. 83, taf. 5. fig. 2. 

 t Gottingische Anzeigen, 1852, p, 867- 



