624 RECORD OF CURRENT RESEARCHES RELATING TO 



are distinct, and are divided by a groove into a narrower anterior and 

 a broader posterior portion. What is of considerable value is the 

 observation that a section of the body-cavity is enclosed between the 

 splanchnic and somatic layers of the mesoblast of these lobes. 

 Connecting this with the observations of Kleinenberg on Lumhricus, 

 the author points out that the procephalic lobe of the spider repre- 

 sents the prfe-oral lobe of the Chtetopod larva ; " but the prolongation 

 of the body-cavity into it does not necessarily imply that it is equivalent 

 to a post-oral segment." There is not yet any trace of the separation of 

 the ganglionic portion of the epiblast of the lobes from the epidermis. 



As the embryo takes on the characters of the adult, the hitherto 

 simple dorsal region begins to be developed, so that there soon 

 appears a ventral instead of a dorsal flexure of the embryo. After a 

 time the heart becomes evident, taking its origin from a solid cord of 

 cells, which are derived from the dorsal mesoblast prior to the 

 diiferentiation of two strata in this region. About the same time the 

 thickenings of the supra-oesophageal ganglia become separated from 

 the epiblast, and the proctodoeum begins to appear. 



Other points of interest are described, but our space requires us to 

 pass on to the general conclusions ; on the whole the history of develop- 

 ment is conclusive as to the closer affinity of the Arachnida to the 

 Tracheata than to the Crustacea (Branchiata). The mesoblast has 

 very much the same history, being in both cases formed by a thicken- 

 ing of the median line of the ventral streak : in the Crustacea the 

 mesoblast is known to be developed from the walls of an invagina- 

 tion. Where mesoblastic somites are found in Crustacea they are not 

 similar to those of the Tracheata. The mesenteron of the Crustacea is 

 formed by an invagination, and the proctodoeum appears before or co- 

 temporaneously with the stomodoeum ; the reverse obtains with the 

 Tracheata, where, too, the mesenteron is not excessively short, nor the 

 proctodoeum very long. It is now almost completely certain that the 

 chelicera3 are true post-oral appendages, and it is clear that, just as in 

 Lumhricus and Peripatus, there is no invagination of epiblast in the 

 region of the ventral nerve-cord. 



In a postscript to this paper, which appears on p. lOfi of the new 

 ' Studies from the Morphological Laboratory, at Cambridge,' Mr. 

 Balfour states that his attention has been directed to the German 

 abstract of a paper, written in Eussian, by Salensky ; from this he 

 gathers that that observer has detected the splitting of the mesoblast 

 into splanchnic and somatic layers, and had given a very similar 

 accoimt of the development of the heart. With regard to the pro- 

 visional abdominal appendages, the final stages of which Mr. Balfour 

 was unable to observe, Salensky found that the anterior pair gave 

 rise to the pulmonary sacs, while he thought that the third and 

 fourth pairs became the spinning mamillas ; the latter view, at any 

 rate, the English observer is inclined to reject. 



Peculiar Modification of a Parasitic Acarian.* — Amongst a 

 large niunber of insects parasitic on plants, the female ready to lay 



* 'Comptcs Eendus,' xc. (1880) p. 1371. 



