88 DR. C. L. BOULKNGER ON 



(a) at a depth of 10 fathoms in Suez Bay on Antedon serripmna 

 Carpenter, {h) at Ul Shubuk (9 fathoms) from Antedon savignyi 

 Miiller, and from Ophiurids * which lived with their arms twisted 

 round those of the Crinoid. This is not the first time an Ophiurid 

 has been described as the host of Myzostomids : H. L. Clark in 

 1902 (4) called attention to the occurrence of a species t on 

 Asteroceras 'pergaimena Lyman, and remarked that the position 

 of the worms upon the concave underside of the arms made it 

 clear that they had not become accidentally attached to the 

 Ophiurids since the latter were dredged %. With regard to 

 Mr. Crossland's specimens, I think there can be little doubt but 

 that the actively moving Myzostomids had migrated from the arms 

 of the Antedon on to those of the commensal Ophiurids ; it is 

 important to note that Antedon savignyi is most probably the 

 Comatida onuUiradiata Lam., from which Leuckart obtained his 

 type-specimens of Myzostoma costatuni. 



A typical adult individual (cf. PI. V. figs. 1-5) has a flat body, 

 approximately oval in shape ; the breadth is usually greatest 

 anteriorly, the posterior region narrowing slightlj?^ behind the 

 last pair of parapodia. There is no distinct transparent border, 

 the intestinal and uterine branches extending to the body- 

 margin. 



On the dorsal surface a median longitudinal elevation is well 

 marked, running along the whole length of the animal ; it is 

 broadest near the middle of its course. From this median eleva- 

 tion arise 6-8 (usually 7) pairs of primary costse which radiate 

 outwards and terminate on the body-margin. The second to fifth 

 pairs are frequently the most developed, the first and last two or 

 three being less distinct and often incomplete. Between the 

 primary costse are secondarj^ and tertiary ones, as described by 

 v. Graff, their number and arrangement varying greatly. In some 

 individuals the costee are broken up so as to form rows of flat 

 tubercles (PL V, fig. 2). The body is thickest in the region of 

 the median longitudinal elevation and gradually thins down 

 towards the margins. 



According to Mr. Crossland's notes, the colour of the living 

 worms is " greenish white, with white dorsal ridge and two 

 longitudinal grey lines on either side of it." The colour of the 

 preserved specimens varies considerably ; a few individuals kept 

 in a separate tube are of a greenish-grey colour, but the majority 

 are of a dark sepia-brown ; this coloration is probably artificial 

 and due to some dissolved pigment in the alcohol with which the 

 worms were preserved. 



The margin of the body is indented so as to form a series of 

 broad, approximately triangular processes, of which in a well- 



* Not yet identified. 



t This species has since (1906) been described by McClendon (13) under the 

 name oi' Mysostoma japoniciim. 



X Another species of llyzostoma has since been recorded from Ojphiacantlia 

 vivijoara (Kcehler, Bull. Sc. France et Belg. vol. xli. p. 279), cf. also Fedotov (6). 



