192 SUMMARY OF CURRENT RESEARCHES RELATING TO 



definition :■ — Larnellibianchiata wi'.h a flattened sole, buccal tentacles, the 

 atrial opening placed at tbe anterior end of tbe gill-axis, filiform gills, 

 kidney-tubes of uniform histological structure, short nerve-connectives 

 and distinctly separated pleural ganglia, and otocysts with a persistent 

 connection with tbe exterior. The other elements of Pelseneer's defini- 

 tion he rejects as not exclusively characteristic, but these points aie 

 sufficient to justify the claim of the Protobranchiata to be the most 

 primitive of the Lamellibranchs. The two families of the order show 

 well-marked elifferences, the Solemyidae differing from the Nuculidas in 

 regard to the following points: — The dermal musculature is well dc- 

 velopeel and the protractor pedis is absent ; the ventral margins of the 

 mantle folds show extensive union, and there is a characteristic branchic- 

 anal siphon ; there are numerous minute peculiarities in regard to the 

 shell (position of ligament, &c.) ; the buccal processes and tentacles are 

 rudimentary ; the alimentary canal is short and simple ; the ventricle 

 of the heart is greatly elongate;!, and there is a pericardial gland on t he- 

 wall of each auricle ; the gills differ both in size and position from those 

 of the Nuculidaj ; the nephridia have no caeca, no cross communication, 

 and compared with those of the Nuculidas occupy a reversed position in 

 the body. Further, the reproductive ducts open into a different region 

 of the nephridia compared with those of the Nuculidae, and there are some 

 minor differences in regard to tbe anatomy of the nervous system. Many 

 of these points have reference to the burrowing habit of Solemya, and 

 are to he looked upon as adaptations to life in tbe sand, so that the 

 Solemyidae are more specialised forms than the Nuciilida?. This agrees 

 with Pelseneer's conclusions^ but the author is unable to accept the 

 suggestion that ibe former have been derived from the latter. Apart 

 from their adaptive modifications the Solemyidae in some points display 

 greater simplicity than even the Nuculidae, facts which the author ex- 

 plains as resulting from the peculiar nature of the environment, which 

 is such that the animals are in part protected from the struggle for 

 existence. The two families are therefore to be regarded as having 

 originated from a common ancestor of simple and primitive structure,, 

 and hoth retain many primitive characters, though adapted for very 

 different habitats. 



Arthropoda. 

 o. Insecta. 



Segmentation of the Insect Head.*— Mr. J. W. Folsom has studied 

 this question in Apterygota. He upholds the segmental character of 

 the intercalary region between the antennary and the mandibular seg- 

 ment, which in certain adult Collembola bears what are interpreted as 

 rudimentary appendages. The superlingua) (paraglossia of the hypo- 

 pharynx) are also regarded as appendages, and it is stated that there 

 are seven pairs of cephalic ganglia in the embryo of Anurida, a genus 

 of Collembola. Dr. Heymons, in reporting on this paper, points out 

 the need for caution in transferring conclusions drawn from a study of 

 the Apterygota to other insects. 



Role of the Adipose Tissue in Metamorphosis.! — Sig. A. Berlese 

 has studied this question in a large number of cases, and has reached 



* Psyche, viii. (1899) pp. 391-4. See Zool. Cenfralbl., vii. (1900) pp. 32-3. 

 t Rev. Patol. Vegetal., viii. (1899) pp. 1-155 (6 pis. and 42 figs.). 



