74 ESSENTIALS OF ZOOLOGY 



large stomodeum and a small proctodeum, both derived from 

 ectoderm, between which the tube consists of endoderm 

 enclosing the enteron. On each side it has been observed 

 in many cases that a protonephridium is formed by an invagina- 

 tion of ectoderm cells, and these end blindly in solenocytes. 

 In this case they are larval organs concerned with the excretion 

 of the schizocoel, as the primitive body cavity is called. They 

 disappear with the development of the pericardial cavity and 

 of the coelomiducts. The resolution of the mesoderm into 

 mesenchyme and a pericardial cavity is an event of import- 

 ance. The first is related to the primitive space between the 

 ectoderm and the endoderm, and this space is, as has been 

 stated, discharged by the protonephridia. 



The primitive circulation of food and oxygen and waste 

 products in the schizocoel or mesenchyme is in the mussel 

 replaced by a blood circulation. The primitive mesoderm or 

 mesenchyme is resolved into free leucocytes which float and 

 move in the fluid plasma and into blood-vessels and a heart. 

 But in the body, in the mantle and the gills, the blood circu- 

 lates still in indefinite spaces in the mesoderm. The peri- 

 cardial cavity is a secondary body cavity, separated by a 

 distinct epithelium, the coelomic epithelium, from the primi- 

 tive body cavity, and is termed the coelom. It is discharged by 

 a special duct, the coelomiduct, which functions as a kidney, 

 but it is to be observed that its function is dependent upon its 

 relationship to the blood. It is aided in its excretionary func- 

 tion by the pericardial glands, the products of which, however, 

 are discharged by the coelomiduct. During these changes 

 the trochophore is converted into a veliger, characterised by 

 the great development of a ring of cilia in front of the mouth. 

 This ring of cilia, the velum, acts in maintaining the larva in 

 the planktonic state and in collecting food, and it is withdrawn 

 when the larva sinks to the bottom (fig. 27). The larva feeds 

 on plankton or suspended matter, as does the adult. 



The Mollusca are related intimately to the Annelida, and 

 through the Annelida to the Arthropoda. They are related 

 also to a group which, like the Mollusca, are characterised by a 

 growth behind the mouth, which in them all has a blasto- 

 poral origin, and because of this growth we may term them 



