410 THE ANATOMY OF INVERTEBRATED ANIMALS. 



joined, so as to leave only an opening for the exit of the 

 foot, and another behind this, which is termed the branchial 

 opening. The edges of this aperture may be prolonged into 

 a tube, which is termed the branchial siphon. When a La- 

 mellibranch is in its natural element and undisturbed, the 

 valves of the shell gape sufficiently to allow of the free en- 

 trance or exit of water to or from the pallial cavity ; or, when 

 siphons exist, they are fully protruded. The cilia with which 

 the branchiae are beset work in such a manner as to drive the 

 water from the infra -branchial chamber, through the open- 

 ings of the branchiae, into the supra-branchial chamber. From 

 hence its only way of exit is by the cloaca and the anal 

 siphon, when the latter exists. In order to make up for the 

 water thus driven out, a new supply of water enters by the 

 interspace between the lobes of the mantle, which bound the 

 infra-branchial chamber, or by the branchial siphon. These 

 currents may readily be made obvious by allowing a stream 

 of finely-divided coloring matter to pass slowly toward the 

 branchial siphon of a Lamellibranch. It will be seen to be 

 swiftly sucked in, and after a very short time a colored stream 

 will flow out of the anal siphon. The same agency brings 

 the nutritive matters suspended in the water within reach of 

 the labial palpi, by which they are guided to the mouth. 



Whatever form the branchiae may possess, they are sup- 

 ported by a chitinous skeleton, in the form of a partial or 

 complete investment to the transverse branchial vessels. 



The mouth is bounded by lips, the angles of which are 

 usually produced on each side into two labial palpi. Some- 

 times the lips are represented by a circular fold produced into 

 numerous tentacula (Pecten). There are no organs for the 

 prehension or mastication of food. A wide and short gullet 

 leads into a stomach surrounded by the liver, which consists of 

 numerous caeca united into ducts which open into the stom- 

 ach. Very generally a diverticulum of the pyloric end of the 

 stomach contains a transparent rod-like body — the crystalline 

 style. 



The intestine usually makes many convolutions, but, finally 

 reaching the middle line of the dorsal region of the body, it 

 terminates by the anus in the posterior part of the pallial 

 chamber. The heart lies in the region traversed by the termi- 

 nation of the intestine. It consists of an auricle and a ventri- 

 cle, or of a ventricle and two auricles, or may be divided into 

 two separate auricles and ventricles (Area). Aortic trunks 

 distribute the colorless blood to the bodv, whence it is carried 



