DR. T. DAVIDSON OX RECENT BRACHIOPODA, 237 



" sponginous band " yield tlic calcareous particles for thickening tlic valves. The perforations in the 

 valves containing prolongations of the mantle subserve the nourishment of the lining tissue of the shell. 

 There are no calcareous spicules in the mantle of Crania, whicli resembles in general structure that of 

 Lingula and RJiijnchoneUa, but is simpler in character. 



The bod}--walls in Crania are pierced by four orifices, the mouth at the top, the anus below on the 

 axis of the body, and on each side by the oviductal organs. No gastro-parietal bands are present in 

 Crania, and the ilio-parietals are comparatively insignificant. The brachial organs cannot be unrolled 

 in the small space between the gaping valves ; the brachial muscles are extremely rigid, and would 

 restrain free movements. Dr. Joubin maintains that the arms cannot be extruded beyond the shell- 

 margins in Crania. The numerous specimens in captivity under frequent observation for consecutive 

 months never extended their arms nor even the cirri, as figured by Barrett. 



The stomach of Crania is filled with transparent mucus. The presence of Diatoms, the chief 

 nourishment of the animal, was frequently detected, also cells of filamentous pelagic algje, and of the 

 calcareous algae (Melobesia) common on the coast, fragments of Radiolarians and of sponge-spicules. 

 The rectal portion of the alimentary canal, which is voluminous and well defined, extends from the great 

 adductor muscles to the median muscle (Hancock's mesenteric). The non-lateral position of the anal 

 orifice, which is situated in Crania between the two posterior adductors exactly in the median line, is a 

 unique feature among the Braehiopoda, and recalls the analogous position of that organ in the Bryozoa. 

 There is no trace of the existence of a heart or arterial system. All the organs are bathed in the colour- 

 less liquid filling the perivisceral cavity. The rudimentary nervous sy.stem is divisible into two sections — 

 the brachial ganglion, whence radiate the numerous nerves of the arms, and a minute peri-cesophageal 

 collar with a slight dorsal enlargement = the cerebral ganglion of ^Mollusca. The two centres are 

 connected by nerves passing down each side of the oesophagus to the base of the arms, and this union 

 leads Dr. Joubin to believe that arms may be regarded as an organ of sense, those of smell and taste being 

 localized in the ciliated brachial gutter at their base near the mouth *. No organs of sight or hearing are 

 present. Dr. Joubin maintains that the sexes are distinct in Crania. There are six genital organs, 

 three on each side of the body, one on the ventral and one on the dorsal, and the third is situated on the 

 perivisceral cavity. The oviductal functions of Owen's so-called " hearts •" and of Morse's segmentary 

 organs is demonstrated. 



From a comparative study of the articulate and inarticulate Braehiopoda, it becomes evident that 

 the pedunculated Lingula and Discina with horny shells arc much more closely related to each other 

 than the non-pedunculate calcareous Crania, which appears to have affinities with the articulate Rhyn- 

 chonella and Thecidium. Crania seems to be intermediate in character between the two great groups, 

 and the position of the anus in the median line distinguishes it not only from the rest of the Tretenterata 

 but from all other Braehiopoda. While fully recognizing the structural differences between these two 

 groups. Dr. Joubin maintains tliat Crania and Rhynchonella each present in a diflFereut way characters 

 which are common to both. The Braehiopoda, he concludes, approach the Bryozoa more than any other 

 roup of organisms, and are sufficiently distinct to form a class absolutely independent of all otiier 

 animals. [A. C] 



b 



» 



* Prof. J. W. Sollas has recently adduced evidence (Proc. Roy. Dublin Soc. 1SS7, pp. .jlS-^SO) to sliow tliat the o;ecaI 

 processes are sense-organs. They are obviously composed of epithelial cells, and exhibit traces of an axial fibre continuous 

 with the nerve-celLs of the mantle. 



