fHE GASTRAiiA-THEORY, ETC. 33S 



logy of the " shell-glands " of the lower Crustacea among the 

 Arthropoda (and the " green glands " of the Decapoda) with 

 the primary kidneys of the Vermes. The Tracheata have 

 quite lost this excretory organ, and the Malpighian tubes 

 of the intestinal canal have taken its place. If we regard 

 the primary kidneys as originally (phylogenetically) in 

 this manner separated skin-glands, it also explains their 

 originally superficial position in the vertebrate embryo. 

 They are here undoubtedly derived from the upper germ- 

 lamellse, either directly from the horny layer or indirectly 

 from cells of the " axial cord," which have passed from the 

 horny layer into the dermal fibrous layer. 



The dermal muscular layer, or the dermal fibrous layer 

 (the " flesh-layer " of Baer, the dermal layers and primary 

 vertebrate layers of Remak), is, as a whole, in its original 

 simple commencement, probably homologous in all the six 

 branches of the Metazoa, or certainly, at least, in the five 

 phylse of the Bilateria. It has probably originated in the 

 Vermes, as well as in the Zoophyta (Hydra, &c.), from the 

 upper germ-lamellcE, and has been inherited from the Vermes 

 by the four higher groups of animals. The corium and the 

 muscular dermal sheath are to be regarded as the two earliest 

 products of its subdivision ; both are perhaps of the same 

 origin, and therefore homologous within the five higher 

 phylse (the Bilateria). The muscles of the trunk of the 

 Vertebrata also proceed from this layer. 



On the other hand, the skeleton system in the different 

 groups of animals is not homologous. Both the internal 

 skeleton formations of the Zoophytes, as well as those of the 

 Echinodermata and the Vertebrata, are entirely different 

 formations, peculiar to each phylon, although all three 

 appear to originate from the dermal fibrous layer. 



The external skeleton of the Vermes and Arthropoda, 

 which is only a chitinised dififerentiation of the epidermis 

 (the so-called hypodermis or chitinogen membrane), as well 

 as the calcareous shells of the MoUusca (also exudations from 



described by J. Miiller, which runs on each side in the folds of the skin of 

 the ventral surface (immediately at the outer surface of the sexual glands), 

 and which opens externally behind on both sides of the Porus abdomhialis, 

 is perhaps to be considered as a homologue, or as a rudiment of the original 

 primary kidneys. (A second further opening in the mouth-cavity is pro- 

 blematical.) If the comparison of this dermal canal of Amphioxus (fig. 40, 

 PI. I, of J. Miiller's work) with the primary kidneys of the Vertebrata, 

 and with the similar excretory organs of the Vermes, were correct, this 

 would establish a very interesting connection between the two latter sets of 

 organs, and would at the same time explain the origin of the passage of the 

 primary kidneys in the Vertebrata from the outer germ lamella. 



