598 



COMPARATIVE ANATOMY 



CHAP. 



the centre of the inter-stomatal region. If we describe the mouth 

 as lying posteriorly and ventrally to the stalk of the buccal shield, 

 the central nervous system lies exactly opposite, that is, posteriorly 

 and dorsally to the same structure. 



Twelve tentacles rise dorsally at the base of the stalk of the 

 buccal shield, six to the right and six to the left of the central nervous 



system. The nervous system 

 extends into the dorsal hypo- 

 dermis of these tentacles (Fig. 

 471). The tentacles are large, 

 are feathered on both sides like 

 the feathers of birds, and are 

 knobbed at their free ends. 



Two pores, plaeed symmet- 

 rically to the inter-stomatal 

 median line, break through 

 the most anterior part of the 

 central nervous system. There 

 is thus open communication 

 between the body cavity of the 

 buccal shield (the proboscis) and 

 the exterior (proboscis pores). 

 Between the central nervous 

 system and the anus there is on 

 each side an aperture. These 

 two apertures belong to the 

 ovlduets. 



Outside of the inter- 

 stomatal region, but in its 

 immediate proximity, the 

 following parts are found. 



Immediately behind the 

 mouth, covered by the oral 

 disc, a thin lamella hangs ven- 

 trally and laterally down from 

 the body like an apron ; this 

 is the postoral lamella (Fig. 

 471, 5). In the posterior 

 angle formed by this lamella 

 with the body, a pore is found 

 on each side (collar pore). 

 These pores lead into the paired middle body cavity, of which we shall 

 speak later. Immediately behind these pores, and like them over- 

 arched by the lateral folds of the postoral lamella, two branchial pores 

 lead from without into the pharyngeal section of the intestinal canal. 

 Musculature. — From near the mouth, longitudinal muscle fibres 

 run back along the ventral side, and enter the pedicle. Muscles are 



Fig. 471, — Horizontal section through Cephalo- 

 discus (after Harmer). 1, Tentacles ; 2, nervous 

 system ; 3, anterior paired body cavity (collar ccelom) ; 

 4, collar pore ; 5, folds of the anterior trunk region, the 

 postoral lamella ; 6, branchial pores ; 7, mesentery ; 

 8, paired trunk ccelom ; 9, pharynx ; 10, cesophagus ; 

 11, stomach ; 12, hind gut. 



