APLYSIA. 207 



by the base of the parapodia round to the left. By slitting 

 up the connective tissue which holds the visceral mass in place 

 the whole of the latter can be lifted and turned backwards 

 and forwards without disarranging the organs.] 



The digestive organs of Aplysia consist of a buccal mass 

 bearing an odontophore and receiving the discharge of two 

 salivary glands ; an oesophagus which dilates into an enormous 

 crop ; two " triturating stomachs " ; a small chamber, the true 

 stomach, into which open the ducts of a voluminous digestive 

 gland or " liver " ; a caecum ; intestine and rectum. The 

 mouth is anterior, the anus postero-lateral and on the right 

 side of the body. (Vide External Characters.) 



Buccal Mass or Pharyngeal Bulb. This is a pear-shaped, 

 highly muscular organ which can be partly thrust out during 

 feeding in order to grasp the weed upon which Aplysia browses. 

 At the anterior narrow end lies the mouth, a median vertical 

 slit-like aperture without lips or distinct outline. The opening 

 is beset with from twelve to twenty longitudinal ridges which 

 allow of considerable expansion in this region. (Fig. 9, buc.f.) 

 The epithelium of the labial region is ciliated and glandular. 

 The mouth leads into the cavity of the buccal mass. This is 

 shown in section in Fig. 9. Its swollen posterior end forms a 

 cul-de-sac on the ventral side and lodges the odontophore. 

 Dorsally is given off the oesophagus, and the ducts of the 

 salivary glands enter the bulb. 



The whole apparatus is held in place by means of stout 

 ligaments whose position and appearance vary with the state 

 of contraction of the animal examined. Some are transverse, 

 some obliquely placed ; all are attached to the body wall round 

 the buccal orifice. They form the extrinsic muscles of the 

 buccal mass. Viewed from the side the buccal mass presents 

 the following groups of intrinsic muscles : — 



1. Paired fan-shaped antero- lateral muscles, originating 

 from the mouth region and spreading out to be inserted on the 

 swollen posterior wall of the buccal mass. 



