ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 179 



median invagination of the floor of the atrium. When the blastopore 

 closes, the endoderm forms a diverticulum, morphologically comparable 

 to a rudimentary notochord. The mesenchyme arises from a Asym- 

 metrical posterior thickening of ectoderm. The primary mesoblasts 

 form two bands, which give rise to three pairs of somites — the anterior 

 forming the embryonic excretory organs, the median and posterior 

 represent the primordia of ovaries and testes respectively. 



Structure of Cephalodiscus.* — A. Schepotieff upholds the position 

 of Mcintosh that Cephalodiscus and Rhabdopleura are nearly allied. 

 The latter is the more primitive, since it has an open " branchial- 

 groove," while the former has two gill-clefts. With Ray Lankester thb 

 author agrees in maintaining that the Pterobranchia form a natural 

 group. They lead on to Enteropneusts, and probably originated from a 

 trochophore type. 



Peculiar Variation of Terebratula transversa Sowerby.f — H. W. 

 Shimmer describes a peculiar specimen, which differs from the type in 

 its more elongate form, thicker valves, obscurity of plications, and less 

 marked development of the mesial sinus in the brachial valve. It is 

 • also remarkable for its extremely rugose surface due to the great 

 inequality in the development of growth lines, and also for the greatly 

 abraded umbo of the pedicle valve. 



Devonian Spirifers.J — F. Chapman gives a note on the identity of 

 Spirifer kevicosta McCoy, with S. yassensis de Koninck, together with a 

 description of S. hoivilti sp. n., from Yictoria. 



Rotatoria. 



Retro-Cerebral Organ in Rotifera.|| — P. de Beauchamp discusses 

 the nature of this organ in Rotifers, which has usually been called the 

 brain-sac by older students of this group. It is situated above and 

 behind the true brain, and is more or less connected with it, but is not 

 part of the brain itself. The author describes three types : The first 

 type is that which is found in Euchlanis dilatata, where it is a very large 

 sac, extending posteriorly over the mastax and anteriorly to the corona, 

 and opens in two short papillse on the head and within the pre-oral 

 ciliary wreath. The wall of this organ is very thin, and contains a 

 secretion which can be stained with neutral red in the living animal. 



The second type is found in Notommata aurita, where the organ is 

 a small sac closely adherent to the under part of the brain, of which io 

 has hitherto been considered to form a part, sending a tubular prolonga- 

 tion upwards, which, after bifurcating, terminates in two openings on 

 the top of the head on a level with the integument. Posteriorly the sac 

 contains small opaque granules which have been erroneously described 

 as calcareous masses. 



* Bergens Mus. Aarb, No. 8 (1905) pp. 1-20 (2 pis.). 

 t Amer. Nat., xxxix. (1905) pp. 691-3 (5 figs.), 

 j Proc. Roy. Soc. Edinburgh, xxv. (1905) pp. 1092-1167. 

 § Comptes Rendus, cxli. (1905) pp. 961-3. 



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