314 CEYLON PEAEL OYSTER REPORT. 



the typical six. Two others have seven such hooks in addition to the simple hook. 

 Meristic variations affecting a specialised organ composed of a limited number of parts 

 are of considerable interest, especially in view of the analogy presented by Eupomatus 

 exaltatus in this respect. Compare also the thoracic segments of Sabellastarte indica. 

 Examples from East Cheval show six, seven and eight hamuliferous hooks respectively, 

 in addition to the main hook. 



The collar-margin is plain ventrally, not projecting forwards between the gills. 

 The anterior free flap of the collar (seen from below) can be distinguished by a 

 transverse groove from the posterior portion of the buccal segment, and at the sides 

 of this groove there is a pair of thoracic organs analogous to those observed in 

 Pomatostegus actinoceros. The orifice is guarded by a small triangular papilla which 

 occurs at the level of the incisura lateralis of the collar. 



Eupomatus minax (Grube). 



Locality : South-west Cheval Paar. 



Small involved round tubes showing coarse growth-rings ; pearl-oyster byssus 

 sometimes attached to the tube. The marginal spines of the opercular disc vary 

 between 19 and 21. The columellar spines are more erect than in Grube's figure 

 ('Ann. Semp.,' 1878, p. 269): the large dorsal spine is vertical and has a strong 

 recurved hook and two lateral accessory hooks. 



Pomatostegus actinoceros, Morch Plate VIII., figs. 3 and 4. 



Morch, Otto A. L., 'Revisio critica Serpulidarum,' Copenhagen, 1863, p. 400, plate xi., 

 fig. 16 ; Grube, 'Ann. Semp.,' 1878, p. 271. 



This is an extremely variable species in regard to the structure of the operculum, 

 and on the other hand it is the exact Indo-Pacific counterpart of the Antillean species 

 Pomatostegus stellatus, Abildgaard (see Ehlers, ' Florida-Anneliden,' 1887, p. 296). 



This species and Eupomatus hetc.rocerus are the most abundant Serpulids in 

 Professor Hekdman's collection, and they must play an important part in consolidating 

 the pearl banks as well as serving as a base for the attachment of the pearl oysters. 

 This is proved by the byssi which still cling to many of the calcareous tubes secreted 

 by these worms. The tube is massive, coarsely rounded or trigonal, rugged and 

 overgrown. Sometimes the tube is provided with a high laciniate keel. The branchiae 

 are spirally rolled and banded. Their appearance varies according to the preservation. 

 The transverse striation sometimes indicated is due to contraction, which causes 

 wrinkling of the strong cuticle which covers the outer surface of the gills. The 

 radioles have an internal septate structure, but the superficial wrinkles are not related 

 to it, and the surface may be quite smooth. In one specimen from the Muttuvaratu 

 Paar, brought up by the divers, I counted 36 radioles in one of the gills. The thoracic 

 uncini showed as many as twelve teeth in addition to the basal, transversely expanded, 



