MYZOSTOMIDA FilOM THE KEl) SEA. 



87 



Myzostoma costatum F. S. Leuckart. (PI. V. & PI. VIII. figs. 2-4.) 



M. costatum Leuckart (10), p. 612; (11), p. 8, pis. i.-ii. 



V. Graff (7), p. 11, pi. i. figs. 13-14; (8), p. 32. 



Myzostoma costatum was one of the first three species of the 

 genus described by F. S. Leuckart in 1830 (10), his account being 

 based on specimens obtained in the Red Sea from Comatula multi- 

 radiata Lam. 



A more complete description of these specimens was published 

 by the same author in 1842 (11), together with the following 

 brief diagnosis of the species : — 



" Char, specif. : Corpore depresso, ovali, margine crenulato, 

 dorso costato ; acetabulis suctoriis et hamuliferis separatis, ace- 

 tabulis utrinque quatuor et hamulis in utroque latere quinque. 

 Hab. in mari rubro, Comatulse multii^adiatse parasitus." 



The species was rediscovered by Semper, who obtained two 

 specimens from Bohol, in the Philippines, and these were de- 

 scribed by V. Graff in his well-known monograph published in 

 1877 (7). The two individuals, which measured 3^ and 2| mm. 

 in length respectively, agreed closely with those described by 

 Leuckart. The dorsal surface presented the characteristic rib- 

 like elevations which gave the name to the species, and, more- 

 over, the animals possessed another feature noticed by Leuckait, 

 namely a small triangular incision at the anterior end of the 

 body, at the apex of which the mouth is situated, v. Graff's 

 diagnosis of M. costatum was as follows: — "Corpus ovale de- 

 pressum, incisura antica triangulari, griseo- aut nigro-brunneum, 

 supra costatum. Costa una longitudinalis mediana qua commu- 

 nicantes 10-14 costse transversales ad marginem producta^. Mar- 

 gine orientes inter has costse breviores secundariee et tertiarse 

 illam longitudinalem non attingentes marginemque crenulantes. 

 Parapodia crassitudine uncinorumque forma M. glahrum, longi- 

 tudine M. cirriferuvi sequantia. Acetabula elliptica. Os ad basin 

 incisurfe triangularis, ventrale, papilla cloacalis ventralis. Longit. 

 ad 3| mm. In mari rubro Comatulse multiradiatae Lam., in mari 

 prope Bohol Actinometra Solaris Lam. incola." 



A few years later v. Graff obtained another specimen, collected 

 by Haeckel at Tur, near Sinai, and described it in the ' Challenger ' 

 Expedition Report (8). This specimen was of a yellowish-brown 

 colour and measured nearly 2 mm. in length. The breadth was 

 rather greater than the length, and the anterior notch, as well as 

 the " ribs," were not so distinct as in the specimens from Bohol ; 

 the marginal notches were, however, more marked. 



As mentioned above, of the large number of Myzostomids col- 

 lected by Mr. Crossland more than 160 belong to a species which, 

 although differing somewhat from previous descriptions, I have 

 decided to refer to Leuckart's 1/. costatum. The series of speci- 

 mens is a most interesting one, containing individuals of all sizes 

 between 0-5 and 4 mm. in length and presenting a wide range of 

 variation. 



Mr. Crossland's specimens were obtained from two localities : 



