ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 995 



from mid-gut to fore-gut, where the epithelium of the former is re- 

 placed by a chitiuous coat, and other changes occur, do not admit of 

 summary, while the structure of the fore-gut has been thoroughly 

 studied by previous investigators. 



Frenzel proceeds to review the various tissues — connective, mus- 

 cular, and epithelial. 



1. The connective tissue. Since the chitin serves to a large extent 

 the same function as is elsewhere discharged by connective tissue, 

 the development of the latter in the Crustacea is relatively small, and 

 its character as supporting tissue is not well marked. Frenzel dis- 

 tinguishes three kinds : (a) fibro-cellular, where from the cells fibres 

 originate, becoming more or less predominant, remaining loosely 

 connected or girt together into a firm network, with interspaces or 

 blood-lacunae ; (b) fibrous, where the lacunae have disappeared and 

 the fibres, which alone remain, have been drawn close to one another ; 

 (c) membranous or elastic, forming a completely closed membrane, 

 probably permeable only by fluids. 



2. The muscular tissue, of cross-striped fibrils, is in no way 

 peculiar. 



3. The epithelial tissue, (a) The intestinal (salivary) glands of 

 the fore- and hind-gut, which were discovered by Braun, resemble 

 ordinary salivary glands both in structure and secretion. A number 

 of cells form a round acinus with a central canal. The cells are 

 sometimes markedly more granular towards the lumen, and the 

 nucleus lies always at the broad basal end. 



(b) The hypodermis (matrix, chitiuogenous membrane). This 

 epithelium, which secretes the chitin, is noteworthy on account of its 

 extreme variability, exhibiting sometimes most beautiful cylindrical 

 cells, and in other species a hardly recognizable cellular character. 

 The cells never exhibit any contents which could be regarded as 

 absorbed food, so that the theory of their possible efficiency in this 

 direction does not seem to receive corroboration. 



(c) The epithelium of the mid-gut consists of well-developed cylin- 

 drical cells, whose regularity is disturbed only by the occurrence of 

 small villi, &c., and by mutual compression. The cells are markedly 

 granular, sometimes extremely fine at the top, but decreasingly so in 

 the middle and lower third of the cell, and becoming very coarse at 

 the base under tlie nucleus. The very varied shapes and the frequently 

 enormous size of the nuclei are remarkable. The cell-border is in 

 some cases resolvable into rows of small bristle-like rods, expanded 

 at their bases, and uniting so as to form a membrane. 



From this detailed histological investigation Frenzel goes on to 

 discuss the proljlera of the regeneration of the epithelial cells of the 

 mid-gut. Though the secretion of digestive fluid in the mid-gut has 

 been mainly transferred to the liver, there are epithelial m'd-gut cells 

 which yield up their whole mass as a S(^cretion, and are in turn 

 replaced by others. The simplest mode of replacement is exhibited 

 when one of the small colls lying next the tunica propma simply 

 divides ; sometimes, however, the cells grow up first for some distance 

 among the epithelial cells and then divide, but frequently the division 



