A REVISION OF THE BEANCHIOSTOMIDiE. 309 



join the ventral portion of the rostral fin. The left metapleur is 

 not continued forward into the rostrum, nor more than a short 

 distance behind the atriopore. The gill-slits are numerous 

 and developed on both sides of the pharynx : they do not in 

 the adult present any numerical relation to the metamerism 

 of the myotomes ; they open into what is in the embryo a 

 median ventral groove, which subsequently becomes closed in 

 as a canal extending over the anterior two thirds of the ventral 

 wall, and opening by a pore (the atriopore) posteriorly. By 

 later enlargement this canal expands into a considerable cavity 

 (the atrium), separating a lateral region of the body on either 

 side (the epipleura) from the axial region; by similar growth 

 it is extended posteriorly within the body as the single or double 

 post-atrioporal caecum or cseca of the atrium. The gonads are 

 developed in either or both of the epipleura. A prseoral 

 muscular hood is developed in front of the mouth, provided 

 along its circular margin with numerous tentacles, supported 

 by a cartilaginoid skeleton ; there is one median unpaired 

 tentacle in the median ventral line, and from ten to twenty 

 (according to age and species) on either side. 



A single circular area of pigment — the eye-spot — is developed 

 on the inner surface of the median anterior termination of the 

 hollow myelon. A single laterally placed olfactory pit may be 

 present on the left side of the snout, consisting simply of a 

 tubular depression of the epidermis by which it is brought into 

 continuity with a slight upgrowth of the terminal wall of the 

 myelon. The anus is placed on the left side of the ventral fin. 



A forwardly projecting csecum (the hepatic caecum) is 

 developed on the right side from the alimentary canal at the 

 point where the pharyngeal perforations cease. 



The oral sphincter (buccal apparatus) is provided with a series 

 of long sensory intra-buccal cirrhi, which project backwards 

 into the pharynx. 



The gonads are developed in a numerous series of coelomic 

 pouches, corresponding in number and position to the myo- 

 tomes of the mid-region of the body. There are no genital 

 ducts. 



VOL. 37, PART 3. — NEV7 SEE. X 



