ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 583 



unnoticed. These characters, when taken with the peculiarities of arthro- 

 pod organization — the feeble development of the somites, the apparent 

 absence of nephridia, the vascular character of the pericardial cavity, and 

 the possession by the heart of lateral ostia opening into the pericardium — 

 are of great morphological interest. 



Developing, later on, the general results which follow on the pseudo- 

 coelic character of the body-cavity, Mr. Sedgwick defines a coelom as a 

 cavity which (1) does not communicate with the vascular system, (2) does 

 communicate by nephridial pores with the exterior, (3) gives rise by its 

 lining to generative products, and (4) developes either as one or more 

 diverticula from the primitive enteron, or as a space or spaces in the un- 

 segmented or segmented mesoblastic bands. Now the vascular space has 

 none of these characters ; it developes either from the blastocoele or from a 

 system of channels hollowed out in the mesodermic tissue of the body. In 

 Annelids and Vertebrates the two spaces co-exist ; in the Arthropoda it is 

 very probable that the coelom persists as the gonads and their ducts, but 

 for the most part vanishes, giving rise possibly to glands of a doubtful 

 nephridial nature, while the body-cavity and vascular system have an ex- 

 clusively pseudoccelic origin ; in the Mollusca the coelom and vascular 

 space have not been sufficiently distinguished from one another ; there 

 seems, however, to be no doubt that the pericardial cavity of the Lamelli- 

 branchiata and Gastropoda represents the entire coelom, for it is always 

 shut off from the vascular system, and it communicates with the exterior 

 by a pair of nephridia. The considerations urged as to the distinctness of 

 coelom and vascixlar system do not, at present, seem to apply to the 

 Nemertinea or Hirudinea. 



In most animals tlie vascular space or pseudocoele appears before the 

 coelom, but in Peripatiis the coelom appears fii-st; in arthropods, at least, 

 the vascular space is in the early stages very commonly occupied by yolk, 

 while the coelom is entirely free from it ; there may be, therefore, some 

 connection between the vascular and the enteric spaces. 



The true coelom of Peripatus appears in the ordinary manner as a series 

 of cavities, one in each mesoblastic somite ; these, which are at first ventro- 

 lateral in position, soon acquire a dorsal extension, and the contained cavity 

 becomes divided into a ventral part, which passes into the appendage, and 

 a dorsal part, which comes into contact but does not unite with its fellow 

 of the opposite side on the dorsal wall of the enteron. The dorsal portions 

 soon become obliterated in the anterior part of the body, but posteriorly 

 they unite with those of their own side to form the generative tubes. The 

 ventral portions retain their isolation throughout life, and give rise to a 

 coiled tube, which is the nephridium of the adult, and a small vesicle 

 which is contained in the appendage, and constitutes the internal blind end 

 of the nephridial portion of the somite. The body-cavity consists of four 

 divisions — a central compartment which contains the intestine and gonads, 

 a pericardial cavity, lateral compartments containing the nerve-cords and 

 salivary glands, and the portion in the appendage. 



If it is true, as is likely, that the coelomic relations of other Arthropods 

 are similar to those of Peripatus, we may add to the definition of the group 

 the terms — coelom inconspicuous, body-cavity consisting entirely of vascular 

 spaces ; while in Vertebrates and most Annelids the body-cavity is entirely 

 coelomic, and the vascular spaces are broken up into a complicated system 

 of channels, and in Mollusca generally the pericardium alone is coelomic, 

 and the vascular spaces are represented by the heart and the more or less 

 complicated system of spaces in the body. 



Mr. Sedgwick enters with some detail into the incomplete segmentation 



