Introduction to Annual Morphology. 



163 



Fig. 22. 



'a semicircle of many e}-es (Hemicyclia). 4. Dirhagea — with 

 two grooves, which may be very long (Nemertes, Linens), 

 or short, with four (Tetrastemma'*''), or no eyes (Meckeliaf ). 

 5. Tetrarhagea — with four grooves (which are converging 

 x-like (Ophiocephalus), or short, transverse (Loxorrhochma). 

 ^Sir/^wZ/z^andJ/t/w^J/^ divide Nemerteans into two sub-orders: — 



1. Anopla — with no style in the proboscis; the nerve trunk 

 placed between the muscular 

 layers ; the mouth opening be- 

 tween the commissures ; deve- 

 lopment with metamorphosis. 

 This includes : — i. Lineidae — 

 with elongated ganglia ; the 

 body muscles externally longi- 

 tudinal, mesially circular, and 

 internally longitudinal ; the 

 proboscis has no internal, 

 longitudinal nor reticular layers, 

 but consists of five coats, ex- 

 ternal elastic, external longi- 

 tudinal, circular, basement, 

 and glandular laminas ; the vessels in the oesophageal wall 

 form a rete mirabile ; the head fissures are deep ; a separate 

 longitudinal band is detached from the investing stratum of 

 longitudinal fibres. Lineus has a long, slit-like mouth and 

 variable eyes ; Borlasia a slender proboscis ; Cerebratulus a 

 flattened body, thinning to the edge, and a cross-like arrange- 

 ment of fibres at each pole of the proboscis in transverse 

 section ; Micrura and Meckelia are also included herein. 



2. Carinellidse — with the lateral nerves between the base- 

 ment and the outer circular layers, and no head grooves nor 

 medio-dorsal vessel ; it includes Carinella, with a short mouth 

 and wide snout ; and Valencinia, with a long mouth and 

 narrow snout. 3. Cephalothricidas have a wide interval be- 

 tween the nerve commissures and the proboscis, armed with 



Diagram of Bipinnaria (p. 136) (B), and 

 a worm (C) ; A.c, anterior ciliary crown ; 

 VI, mouth ; 7n' , stomach ; P.c, posterior 

 ciliary crown ; a, anus. 



* Tetrastemma has four eyes in a square. In Prosorhochmus, they do 

 not form a square. 



t Meckelia has feeble or no head fissures. 



]M 2 



