944 SUMMARY OF CURRENT RESEAECHES RELATING TO 



Are the Tunicata degenerate Pishes ? * — Prof. E. van Beneden disr 

 cusses the arguments of Prof. A. Dohrn in favour of the degeneration of th© 

 Tunicate from fishes. He regards those arguments as based on the belief 

 that the pseudobranchial grooves of the Cyclostomata are derived from a 

 pair of branchial clefts, and that the rudiment of the thyroid of fishes, the 

 hypobranchial organ of the Cyclostomata, the hypobranchial band of 

 AmpJiioxus, and the endostyle of Tunicates, are the modified remains of 

 another pair of branchial clefts. But the study of the innervation of the 

 branchial apparatus of Ammocoetes shows that the first branchial cleft of 

 the Cyclostomata is the homologue of the spiracle of Selachians, and the 

 true branchial nerves of one have just the same disposition as those of the 

 others. If its innervation is to be the criterion the thyroid body represents 

 several segments. 



In the answer which Prof. Dohrn has made to these criticisms he 

 denies the statements of M. Julin as to the innervation of the branchial 

 apparatus of Ammoccetes. Prof, van Beneden points out that the German 

 naturalist has studied very small larvae, whereas Julin examined such as 

 had nearly completed their development. M. Julin is to investigate young 

 forms in order to control the observations of Dohrn. In a further answer 

 Dr. Dohrn refers merely to a slight criticism of Prof. Beneden. 



Arthropoda. 



Structure of Alimentary Canal.f — Prof A. Schneider communicates 

 a series of notes on the anatomy and histology of the alimentary canal of 

 Arthropods. 



(1) The hypodermis of insects consists, as Schneider and others have 

 previously maintained, of a nucleated protoplasmic layer, without distinct 

 cells, continuous with the sarcolemma and neurilemma of muscle and 

 nerve, a literal ecto-mesoderm. Chitin is not an excreted substance, but a 

 slow modification of the protoplasm. There is no real difference between 

 that formed from muscle insertion, and that formed from the protoplasm. 



(2) Fore- and hind-gut have the structure which one would expect in 

 invaginations of the ectoderm, — internally a chitinous layer, not sharply 

 defined from the outer homogeneous hypodermis, which is succeeded by a 

 layer of transverse and longitudinal muscle-fibres, the sarcolemma of which 

 is continuous with the hypodermis. The ridges of the hind-gut of cater- 

 pillars are formed from longitudinal muscle-fibres. 



(3) The mid-gut exhibits (a) the cellular digestive and absorptive layer, 

 (b) chitinous lamellae, (c) hypodermis, and (dl) muscle-fibres. The chitin- 

 ous layer of this region is renewed like that of other parts during skin- 

 casting. These results hold true of other Arthropods as well as insects, to 

 which they principally refer. 



(4) Special structures, (a) In many insects the hind - gut exhibits 

 spinous modifications of the chitin. These are briefly referred to. (6) 

 Similar chitinous thickenings in the fore-gut are much more frequent. 

 Their disposition in various insects is simply noted. 



(5) Musculature of fore-gut. The fibres are longitudinal and transverse, 

 the latter occasionally radial. No notice has hitherto been taken of the 

 occurrence of what may be called a " proboscis " (Kiissel). Posteriorly the 

 fore-gut is in some cases evaginated forwards and outwards, projecting into 

 the lumen of the mid-gut. This is associated with an alteration in the 



* Zool. Anzeig., x. (1887) pp. 407-13, 433-6, and 582-3. 

 t Zool. Beitr. (Schneider), ii. (1887) pp. 82-96. 



