CCELOM OF MOLLUSCA. 367 



out. In the Cephalopoda, the Dibranchiata are ordinarily provided 

 with an ink-bag, which is an organ of this kind; in many (Loliginidas) 

 this opens together with the hind-gat, and may, therefore, repre- 

 sent a structure which has been developed from it, although, iudeed, 

 in other Cephalopoda, it opens below or behind the anus. It forms 

 an elongated sac, the contractile walls of which form internally- 

 projecting lamella (Fig. 103, a), and which secretes the well-known 

 black fluid. 



Ccelom. 



§ 283. 



A coelom appears very early in the differentiation of the 

 Molluscan body. The complications which affect the coelom, owing 

 to the coils of the enteric canal which are embedded in it, and the 

 appended organs which are developed from its walls, are further 

 increased by the formation of other organs, and, especially, of the 

 generative system ; the cavity is thereby broken up into a number 

 of segments varying in size. As a rule, the coelom is continued into 

 the processes of the body, as, for example, into the mantle-flaps of 

 the Lamellibranch, and the mantle of the Gastropoda. Less im- 

 portant appendages of the body are also frequently connected with 

 the coelom. 



As a rule, the vascular system is freely connected with the coelom, 

 which therefore forms a portion of the haemal system. This 

 arrangement is more or less well marked; and more or less wide 

 spaces are shut off from the ccelom, according to the degree to which 

 the vascular system is developed. When the wider spaces of the 

 ccelom are connected with the vascular system, these portions of the 

 luminal system form lacunas ; when these spaces are broken up, either 

 by the organs which are embedded in them, or by the bands of tissue 

 which connect together the walls of these organs, they become con- 

 verted into narrow, and often into vascular canals, which, moreover, 

 are frequently arranged in a regular manner. In the Lamellibran- 

 chiata and Gastropoda there are all kinds of stages of this kind ; 

 in the Cephalopoda the blood-vascular system is very complete, and 

 true lacunar spaces are, for the most part, confined to the visceral 

 sac. By means of the excretory organs (§ 289), the ccelom, as in 

 many Vermes, communicates with the surrounding medium. 

 The water is thereby enabled to mix with the blood. In 

 addition to the communications which the coelom has with the 

 exterior by means of the excretory organs, it is able to communicate 

 with it directly, owing to the presence of pores in the foot of 

 the Lamellibranchiata and Gastropoda; the fluid is able to 

 escape from the coelom through these pores. This has been definitely 

 made out both in the Lamellibranchiata (Mactra, Cardium, Solen) 

 and in the Gastropoda (Pyrula). This fluid is of especial importance 

 in locomotion, inasmuch as the animal is enabled to swell out its 



