524 SUMMARY OF CURRENT RESEARCHES RELATING TO 



function of acting against the elastic force of the ligament. In forms 

 which have adductors wholly of the "vitreous" type, e.g. Sol en and 

 Latraria elliptica, the closure of the valves, though it may be rapid, is 

 always of short duration. 



JUtheriidse.* — R. Anthony discusses this family of bivalves, which is 

 usually regarded as allied to Unionidse. There are three genera : 

 JEtheria from African rivers, Bartlettia and Mulleria from South 

 American rivers. The first two are Dimyarian, the third Monomyarian. 



In JEtheria, as in oysters, the fixation is by one valve, oftenest the 

 left, and all their peculiarities may be related to this special condition of 

 " pleurothetic " fixation. From among Unionidse this family has arisen 

 probably by adaptation to particular conditions of habitat. Thus the 

 elongated fixed valve of JEtheria caillaudi is advantageous in quiet 

 waters by enabling the animal to raise itself above the level of the floor. 



Arthropoda. 



Structure and Classification of Arthropoda.f — E. Eay Lankester 

 has reprinted the articles Arthropoda and Arachnida which he con- 

 tributed to the tenth edition of the " Encyclopaedia Britannica." He 

 adheres to his formerly expressed conclusion as to the genetic affinity 

 and monophyletic origin of the Chaetopods, Rotifers and Arthropods, 

 for which in 1878 he introduced the term Appendicnlata. The Arthro- 

 poda might be better called " Gnathopoda," since their distinctive 

 character is that one or more pairs of appendages behind the mouth are 

 densely chitinised and turned (fellow to fellow on opposite sides) towards 

 one another so as to act as jaws. This is facilitated by an important 

 general change in the position of the parapodia ; their basal attachments 

 are all more ventral in position than in the Chastopoda, and tend to 

 approach towards the mid-ventral line. Very generally (but not in 

 Onychophora) all the parapodia are plated with chitin secreted by the 

 epidermis and divided into a series of joints, giving the " arthropodous " 

 or hinged character. 



Other distinctive features hold the Arthropoda together, and suggest 

 the view that they have been developed from a single line of primitive 

 Gnathopods, which arose by modification of parapodiate annulate worms 

 not very unlike some of the existing Chaatopods. These additional 

 features include the ostiate heart and the " plileboedesis" that is to say, 

 the peripheral portions of the blood-vascular system are swollen so as to 

 obliterate to a large extent the coelom, whilst the separate veins entering 

 the dorsal vessel or heart coalesced, leaving valvate ostia, by which the 

 blood passed from a pericardial blood-sinus formed by the fused veins 

 into the dorsal vessel or heart. Another feature is that the region in 

 front of the mouth is no longer formed by the primitive prostomium, or 

 head-lobe, but one or more segments, originally post-oral, with their 

 appendages, have passed in front of the mouth (" prosthomeres "), and 

 the brain has become a syncerebrum. Moreover, as in Chaetopods, 

 ccelomic funnels (" ccelomoducts ") may occur right and left as pairs in 



* Comptes Rendus, cxxxviii. (190-4) pp. 1233-5 (2 figs.). 



t Quart. Journ. Micr. Sci., xlvii. (1904) pp. 523-82 (1 pi. and lrfigs.). 



