SHELLS OF ERACHTOPODA 



927 



be considered to be the animal's blood. Of their special function in 

 the economy of the animal it is difficult to form any probable idea ; 

 but it is interesting to remark (in connection with the hypothesis of 

 a relationship between Brachiopods and Polyzoa) that they seem to 

 have their parallel in extensions of the perivisceral cavity of many 

 species of Flustra, Eschara, Lepralia, &c., into passages excavated in 

 the walls of the cells of the polyzoary. Professor Sollas l finds in 

 the centre of these prolongations an axial fibre which can be traced 

 backwards to the nerve-cells of the mantle ; at the distal end is a 

 terminal cell which is connected by a fibril with the axial fibre, and 

 is covered externally by a transparent chitinous layer ; save for the 

 absence (or the unproved presence) of pigment cells we should be 

 justified in regarding the processes as organs which are sensitive to 

 luminous impressions. 



In the family Rhynchonellidce, which is represented by only 

 six recent species, but which contains a very large proportion of 



FIG. 702. 



FIG. 703. 



FIG. 704. 



FIG. 702. Horizontal section of shell of Terebratula bullata (fossil, Oolite). 

 FIG. 703. Megerlia lima (fossil, Chalk). 



FIG. 704. Spiriferina roStrata (Triassic). 







fossil Brachiopods, these canals are almost entirely absent ; so 

 that the uniformity of their presence in the Teretoatulidce, and their 

 general absence in the Rhynchonellidce, supply a character of 

 great value in the discrimination of the fossil shells belonging 

 to these two groups respectively. Great caution is necessary, 

 however, in applying this test ; mere surface markings cannot be 

 relied on ; and no statement on this point is worthy of reliance 

 which is not based on a microscopic examination of thin sections of 

 the shell. In the families Spiriferidce and Strophomenidce, on the 

 other hand, some species possess the perforations, whilst others are 

 destitute of them ; so that their presence or absence there serves only 

 to mark out subordinate groups. This, however, is what holds good 

 in regard to characters of almost every description in other depart- 

 ments of natural history; a character which is of fundamental 

 importance from its close relation to the general plan of organisation 

 in one group being, from its want of constancy, of far less account 

 in another. 2 



1 Proc. Boy. Dublin Soc. v. 318. 



2 For a particular account of the Author's researches on this group see his memoir 

 on the subject, forming part of the introduction of Mr. Davidson's Monograph of the 



