MYZOSTOMIDA FROM THE RED SEA. 93 



The above description of M. costatum refers especially to full- 

 grown specimens measuring between 1| and 4 mm. in length. 

 Young specimens under 1| mm. present a somewhat different 

 appearance ; the body is thinner, and the costje on the dorsal 

 surface are less conspicuous than in the adult ; though in some 

 cases very faint they are never absent. The mai-ginal region is 

 more delicate and ti-ansparent than in the larger specimens, this 

 being due to the absence of ova in the peripheral branches of the 

 uterus ; 12-20paii^sof well-defined cirri are present, and between 

 them can be seen smaller processes which are about to give rise 

 to more of these oi-gans. The last pairs of cirri show the same 

 tendency towards enlargement as in the adults. The suckers are 

 not very prominent, but the male papilite are fully developed and 

 are extremely conspicuous. 



Before concluding my account of the external features of this 

 species, I wish to call attention to the fact that quite a considerable 

 number of specimens in Mr. Crossland's collection present abnor- 

 malities chiefly regarding the number of parapodia. In these 

 soecimens one or more parapodia are missing from one or both 

 sides of the animal, and, although in some cases this may be due 

 to injury and incomplete regeneration of parts of the body, in 

 others the malformations must have been congenital. I have 

 floured some of these abnormal specimens ; the sketch (text- 

 fig. 18) on p. 91, shows an extreme case, that of an individual 

 measuring 1'25 mm. in length, which possesses four parapodia on 

 one side of the body and only two on the other ; the rest of the 

 animal seems normally developed, and the full number of suckers 

 is present. In some cases reduction in the number of parapodia. 

 is accompanied by a similar reduction in the number of the 

 suckers {cf. text-fig. 17, p. 89). 



Alimentarij Canal. 



As mentioned above, the mouth is situated on the ventral side 

 close to the anterior extremity of the body ; through it can be 

 protruded a large and muscular pharynx provided at its distal end 

 with small papillce, as described by v. Graff; these are very in- 

 conspicuous in all the specimens before me, and, so far as I can 

 ascertain, number four only. 



The stomach is elongated, extending back as far as the level of 

 the last pair of parapodial bases, and constricted laterally at the 

 points where the intestinal branches arise, almost opposite the 

 male genital papillse. 



There are only two intestinal ti"unks on each side arising close 

 together from the stomach ; these divide into from 7-9 primarj^ 

 branches, which in their turn radiate outwards and give rise to 

 numerous diverticula, which end blindly on the body-margin. 



The rectum is separated from the stomach by a distinct valve- 

 like constriction, and takes the form of a straight tube running 

 back from the level of the last pair of parapodia to the cloaca, 

 where it is joined by the oviduct. 



