r.CELOM OF VERMES. 165 



§ 135. 



The glandular organs connected with the mid-gut are ordinarily 

 regarded as hepatic, or as a " liver." We must be careful not to attri- 

 bute anything more than the value of a convenient distinction to these 

 names. Separate glands are almost always absent from the mid-gut 

 of the Vermes, but the epithelium is generally found to be different 

 from the epithelia of the other divisions of the enteron, so that a 

 secretory function is not improbable. This is indicated by the 

 granular character of the cells in many cases, and by the difference in 

 the coloration of the cell-contents. The latter point is probably more 

 important than the former, for the former may be due to the absorbing 

 function of the enteric epithelium. The mid-gut of the Bryozoa is 

 distinguished by this character, and even in the Rotatoria the epithelial 

 layer of this region may be seen to be differentiated. This character 

 is more highly developed in the Platyhelminthes (Planarias, many 

 Trematoda), in which the stomachal branches (Fig. 68) are the 

 chief seat of this peculiarity ; these branches, therefore, may be re- 

 garded as secreting appendages. Independent glands are still more 

 distinct in the lateral appendages of the mid-gut of Aphrodite 

 (Fig. 74), which are developed by the gradual narrowing and 

 lengthening of the simpler enteric appendages found in the allies of 

 this genus. Finally, we must mention here the tubular enteric 

 appendages of Balanoglossus, which beset the whole dorsal surface 

 of the enteric canal beyond the respiratory segment, and are 

 grouped in agreement with the segments of the body. 



Ccelom. 

 § 136. 



The Ccelom of the Vermes is the earliest differentiation of a 

 hollow cavitary system placed between the enteric tube and the 

 integument, and leading to the formation of a vascular system, and 

 takes its origin in a cleavage of the mesoderm. The food taken in 

 by the enteron is no longer distributed in the organism, as it is in 

 the Ccelenterata, by continual imbibition into the tissues from the 

 wall of the enteron, but the nutrient fluid is collected into a 

 perienteric space, where it may enter into relation with organs 

 differentiated from the enteric canal, and from the integument. 



In a large number of Vermes this perienteric space (Ccelom) is 

 either altogether absent, or only rudimentarily present. This is the 

 case in most of the Platyhelminthes and Nemathelminthes, as well 

 as in some others, such as Pedicellina. In the land Planarians two 

 cavities traversed by a reticulum of connective tissue extend along 

 the body ; they are largely broken up anteriorly. They are to be 

 regarded as indications of a ccelom of this kind. The ccelom is 



