364 COMPARATIVE ANATOMY. 



(many Opisthobranchiata) . When they are more highly developed 

 their duct is elongated, so that the secreting portion comes to lie 

 some way further back ; sometimes it is placed on the oesophagus, 

 and at others on the stomach itself. In this case the glands are 

 rounded, elongated, and generally flattened tubes (Prosobranchiatn, 

 many Pulmonata), which may be broken up into several smaller 

 parts, or have the form of ramified organs ; the glands found on 

 the stomach of Pleurobranchus are examples of this latter kind. We 

 not unfrequently meet with two pairs, the efferent ducts of which 

 are either separate all along their course, or the ducts of the hinder 

 pair unite with one another. Even when there is but a single pair 

 of glands they may be often observed to fuse into one mass, the 

 double nature of which is shown by the presence of two efferent 

 ducts. The salivary glands of many Prosobranchiata are differen- 

 tiated functionally (Dolium, Cassis, Cassidaria, Tritonium), for part 

 of the gland secretes free sulphuric acid. The glands of some. Opis- 

 thobranchiata (Pleurobranchus, Doris) are differentiated in the same 

 way. 



Among the Cephalopoda, Nautilus is provided with a paired 

 glandular mass, which is placed inside the pharynx. These glands 

 are also present in many Dibranchiata (Octopus, Eledone), as 

 are others, which are short and lie just behind the pharynx; these 

 have an efferent duct which penetrates the wall of the pharynx, and 

 unites with its fellow of the opposite side immediately in the 

 orifice of the duct (Fig. 199, gls .<?). In addition, there are glands 

 behind these which lie at the sides of the oesophagus, and behind the 

 point at which it penetrates the cephalic cartilage. They are either 

 simple or lobate ; as a rule their efferent ducts unite into a single 

 one, internally to the cephalic cartilage, and this duct opens into the 

 pharyngeal cavity in front of the lingual ridge (Pig. 199, gls i). 



Paxceri, P., Gli orgaui e la secrctioue dell' aeido solforico nei Gasteropodi. Atti 

 della K. Accad. dellc Sc. fisichc. Napoli. Vol. III. 



2) Appendages of the Mid-gut. 

 § 281. 



Appended organs are very generally found on the mid-gut of the 

 Mollusca ; they represent the " liver," and are differentiations of the 

 wall of the enteron, from which they are developed, under the form 

 of diverticula, the primary origin of which is the sacculation of the 

 endoderm. 



In the Lammellibranchiata the liver is a gland which sur- 

 rounds the stomach and a large portion of the rest of the enteron. 

 It forms numerous acini, which arc connected together into largo 

 lobes, and open at various points either into the stomach, or into the 

 succeeding division of the enteron. 



