COELOM. 



which though not referred to in the definition, is of considerable 

 importance, and must be shortly dealt with here, and that is the 

 form and development of the Coelom. In the Cephalochorda 

 and Enteropneusta the coelom originates as outgrowths of the 

 primitive gut (archenteron). In Vertebrata, though there is 

 no actual outgrowth of the enteron, the walls of the coelom 

 originate from tissue which is derived from the wall of the 

 enteric space, and there can be but little doubt that the mode of 

 development is referable to the enterocoelic type, found in 

 the two other phyla, and is indeed a modification of it. 



If then we leave out of consideration the Tunicata, which 

 in this respect cannot at present be brought into line with the 

 other chordate phyla, 

 we may assert that 

 an enterocoelic origin 

 of the coelom, or a 

 modification of it, is 

 characteristic of the 

 Chordata. Ou t s i d e 

 the Chordata a similar 

 mode of origin of the 

 coelom is found in the 

 Chaetognatha (vol. i, 

 p. 590), in the Bra- 

 chiopoda (vol. i, p. 

 580), probably in the 

 Phoronidea (vol. i, p. 



546), and as will be shown further on in the Echinodermata. 

 But the coelomic resemblances between these animals go farther 

 than this. In the Enteropneusta the archenteric outgrowths 

 are five in number — two pairs and an anterior unpaired out- 

 growth (Fig. 3). These, following the nomenclature of Bateson, 

 have been named according to the position they occupy in the 

 adult : the anterior unpaired sac is called the proboscis cavity ; 

 the sacs of the anterior pair are the collar cavities ; and the 

 posterior sacs are the trunk cavities. In the Enteropneusta 

 they undergo no further division, but remaining in the parts 

 of the body indicated by their names, they give rise to the 

 coelomic spaces of the adult. 



In Amphioxus the anterior unpaired sac is called the preoral 



Fig. 3. — Diagrams showing the origin and primitive 

 relations of the coelomic sacs A in Balanoglossus, B 

 in Amphioxus (after MacBride). 1 proboscis cavity 

 in A, preoral cavity in S ; 2 collar cavity ; 3 anterior 

 somite of trunk ; 4 trunk cavity. 



