276 COMPARATIVE ZOOLOGY OF CHORDATES 



mesentery suspending the large intestine (mesocolon). This 

 extension, which is called the great omentum, brings about an 

 increase in size of the omental sac, on the wall of which fat 

 is often deposited. 



The Mullerian or oviducts and the uterus are suspended by 

 mesenteries, called mesometria, and which are of interest in 

 determining the relation of the implanted blastocyst to the 

 walls of the uterus. The mesentery supporting the testis is 

 called the mesorchium, that supporting the ovary the meso- 

 varium. 



Ccelomic cavities are always lined by mesodermal tissue. 

 In Amphioxus, the coelomic cavities of the somites, when they 

 arise, are in open communication with the gut, and are hence 

 known as enteroccels. In higher forms, the coelomic cavities 

 appear as splits in the mesoderm, without communicating with 

 the gut. These cavities are known as schizoccels. The method 

 of origin is not of much importance, but it is important to 

 realise that all cavities which arise, either as subdivisions of, 

 or outgrowths from, enteroccels and schizoccels, are ccelomic. 

 So the cavities of the pericardium, of the kidney- tubules, of 

 the Wolffian and Mullerian ducts, of the gonads in Amphioxus, 

 are ccelomic. On the other hand, no cavity is coelomic which 

 does not arise in this way. The cavities of the blood-vessels 

 are not coelomic, although their walls are composed of meso- 

 dermal tissue. Cavities lined by tissue other than mesoderm, 

 such as those of the atrium of Amphioxus, nerve- tube, 

 nephridia, amnion, gut, or blastocoel, are, of course, not coelomic. 



The coelomic cavities originally probably opened to the 

 outside in each segment for the purpose of freeing the germ- 

 cells. Something like this happens in Amphioxus, where also 

 the left anterior head-cavity opens into the preoral pit. Rarely, 

 in higher forms, the cavities of the premandibular somites 

 may open into the hypophysis. Comparable " proboscis- 

 pores " (see p. 360) occur in Balanoglossus and its allies, and 

 in the Echinodermata. In the Craniata, the splanchnocoel 

 may communicate with the outside, through the genital pores 

 via the cloaca as in Petromyzon, through the abdominal pores 

 as in Scyllium, or through the Mullerian ducts. 



