644 SUMMARY OF CUEEENT RESEARCHES RELATING TO 



resemblance between Brachiopods and Chfetopods confined to external 

 characters ; in botb the intestinal canal has the same direct relation 

 to the dorsal wall of the body, and in the adult the canal is attached 

 by two mesenteries. Notwithstanding certain points of disagreement, 

 the muscular system of the two sets of forms ofi"ers some striking 

 characters of resemblance. So, again, the ovaries are in both sus- 

 pended by a mesentery, and the oviducal glands of the Brachiopod are 

 very similar to the segmental organs of worms. As the gills of 

 Brachiopods are always found on the dorsal side of the mantle, they 

 may perhaps be homologized with the dorsal gills of such forms as 

 Eunice and Nerine ; on the other hand, the cephalic segment may, as 

 in Thecidium, enter into the formation of the gills, which might, 

 therefore, be homologized with the gills of the Cephalobranchiate 

 Annelids. The greatest difficulty is to be found in a comparison of 

 the shells, which are not the same as the tubes of some Worms ; on 

 the other hand, the prolongations of the mantle which fill the tubules 

 of the shells present a great resemblance to the prominences which are 

 found in the gelatinous subcutaneous layer of the Chloraemina. On 

 the whole, Kowalevsky is inclined to regard the Brachiopoda as 

 merely an order of the Annelids, for he thinks that they present as 

 many resemblances to the Chaetopods as do, for example, the 

 Discophora. 



Arthropoda. 



Eyes of Ai'thropoda.* — B. T. Lowne points out that three distinct 

 forms of eye are to be found in the Arthropoda, the compound eye, the 

 eimple ocellus, and the compound ocellus, described by Landois as 

 being found in larval insects. The author believes that the relation- 

 ship of the first two is very distant, but that there is a much closer 

 relation between the compound eye and the compound ocellus, the 

 former being merely an aggregation of a great number of these ocelli, 

 variously modified in different forms. There is, further, a fourth kind 

 of eye, which may be regarded as forming an intermediate link 

 between these two, and which may be well spoken of as the " aggregate 

 eye " ; it is to be found in Isopods. 



In the compound eye we find that there is a membrane which 

 separates the crystalline cones and great rods from the more deeply 

 lying nervous structures ; tliis is now called the memhrana hasilaris. 

 It is usually attached to the cornea by an inflected ring of integument 

 — the scleral ring — so that the crystalline cones and great rods are 

 entirely inclosed in a case ; all these structures receive the name of 

 the dioptron, and they apparently correspond to the cornea, vitreous 

 and fibrous membrane of the simple ocellus. Beneath the dioptron is 

 a structure now called the neuroriy which consists of a retina, an optic 

 nerve, and an optic ganglion. 



Lowne states that for several years he has regarded as inadequate 

 all the theories that have been put forward to explain the mode of 

 action of the compound eye ; he has lately observed in the fresh eye of 

 a Pterojphorus that the spindles are, during life, large ovoid bodies, 



* Pioc. Koy. Soc, XXXV. (1883) pp. 140-5. 



