90 DR. C. L. BOULENGER ON 



of tlie body is subject to great variation, and it seems that the 

 development of caudal appendages bears no relation to the size of 

 the individual, these organs being absent in some specimens of 

 considerable size {cf. text-lig. 17), and, per contra^ being well 

 developed in individuals under a millimetre in length. 



The presence of caudal processes, as well as the number of these 

 organs, are characters which have been used to a considerable 

 extent for tuxonomic purposes; the above description, however, 

 shows how unsafe it is to fix the limits of species by them. 



The vential surface is quite smooth ; the five pairs of parapodia 

 have the usual radial arrangement, and their insertions are ap- 

 proximately equidistant between the centre of the body and the 

 periphery. 



The structure of the parapodia is as desci'ibed by v. Grafi" (7) ; 

 they recall those of M. cirriferutn, although differing in being 

 rather more muscular. When fully extended they project as far 

 as the body-margin or even slightly beyond it, their extremities 

 being sometimes visible in a dorsal view of the animal. 



The parapodia seem capable of movement in every direction, 

 and in his field -notes on this species Mr. Ci-ossland remarks that 

 the little animals " move their leg-like parapodia very actively, 

 like the legs of an insect, and can crawl actively abovit the arms 

 of the Ophiuroids and on a needle, but not on glass." 



The hooks are powerfully built, and their tei-minations strongly 

 bent, more so than in M. cin-ifenim, being in this respect inter- 

 mediate between that species and M. glahrum. 



Lateral to the third para podium on either side is a large male 

 genital papilla ; its insertion is close to the base of the para- 

 podium, and when completely protruded it can extend beyond 

 the margin of the body. 



Both mouth and cloaca are ventrally situated close to the ex- 

 tremities of the body. Except in one individual the anterior 

 body-margin in front of the mouth is complete and without a 

 triangular incision. In the specimen figured on PI. Y. fig. 4, 

 such an incision is present, and in this respect it resembles those 

 described by v. Graff and Leuckart. I am, however, inclined to 

 consider this a, post-mortem phenomenon due to the abrasion of 

 the delicate area which I mentioned above as occurring just in 

 front of the mouth. 



In a ventral view of M. costatum the " suckers " are quite con- 

 spicuous, and of these there are usually six pairs, all equally 

 developed and situated close to tlie periphery of the body. Four 

 pairs are in the usual position, that is to say in the interspaces 

 between the five pairs of parapodia, these are the second to 

 fifth pairs ; the first is situated in front of the first pair of paiu- 

 podia, close to the anterior margin, whilst the last or sixth pair of 

 suckers lies approximately halfway between the last pair of para- 

 podia and the posterior median termination of the body. 



The suckers have much the same structure as those described 

 by Wheeler (17) in M. glahrum, and when retracted appear as 



