OPHIUROIDEA 161 



feed. They are also often found clinging to Gorgonians. They 

 reach a considerable size, up to ca. 10 cm. diameter of disk. 



The development is only partially known. The eggs (at least 

 of G. caput-medusce) are laid free in the water, but it is unknown 

 whether they develop into an Ophiopluteus larva. It appears 

 that the young, newly metamorphosed Gorgonocephalus (at least 

 G. eucnemis) lives parasitic on the polyps of the Alcyonarian 

 Eunephthya (Gersemia). At the stage when branching of the 

 arms begins they leave the Alcyonarian and may then seek the 

 adult specimens, on the backs of which the further growth stages 

 are often found. 



The remarkable parasite Protomyzostomum (related to the 

 Myzostoma of Crinoids) lives in the body cavity of Gorgono- 

 cephalus. It enters the gonads, the content of which is destroyed, 

 their walls forming a cyst around the parasite (cf . p. 151). It does 

 not castrate its host, the gonads not directly destroyed by the 

 parasite functioning normally. 



Preserving Gorgonocephalus with the arms well expanded 

 requires a special treatment. Some species are best kiUed in 

 formalin, others, e.g. G. caput-7neduscE , may be very beautifully 

 preserved by putting the weU- expanded specimens into fresh 

 water by which they become completely paralysed, and then 

 transferring them into formalin or alcohol. 



Three species are known from the British seas. A fourth 

 species, Gorgonoc. arcticus (Leach) (S}ti. G. Agassizi Stimpson), 

 occurs in the Arctic seas and at least as far south in the Norwegian 

 sea as 62° 43' N., 1° 26' S. (" Michael Sars ") ; it may well be 

 expected to occur also as far south as the cold area proceeds in 

 the Faroe Channel, and is therefore included in the key. 



Key to the species of Gorgonocephalus known from, or likely 

 to occur in the British seas. 



1. Disk wholly covered with coarse or fine spines ; those on the 



radial ribs not specially prominent .... 2 



Disk naked, only the radial ribs with some larger spines or with 



smooth, rounded grains ...... 3 



2. Disk with rather coarse spines or tubercles ; arm spines smooth, 



curved . . . .1. Gorg. caput-medusce (Linn.) 



Disk closely covered with fine spinelets ; arm spines compressed, 

 coarsely pectinate (Fig. 93, 1) 



3. Gorg. Lamarcki (Miill. andTrosch.) 



3. Radial ribs with rounded grains 



2. Gorg. eucnemis (Miiil. and Trosch.) 

 Radial ribs with larger spines . . . Gorg. arcticus (Leach) 



M 



