24 DR. T. DAVIDSON ON RECENT BRACHIOPODA. 



surface of the auimal, on each side of the median line, elongated in the antero-posterior 

 direction, and extending between the occlusors, almost as far forward as their anterior 

 margins. They are of irregular form, and enlarged posteriorly in T. caput-serpentis. 



In PI. IV. figs. 8 & 9, we give Hancock's two enlarged figures showing the dorsal 

 and ventral views of TerebratuVma caput-serpentis deprived of the shell, and exhibiting 

 the respective positions of the various muscles above described. 



While alluding to the digestive organs, Hancock states {I. c. p. 814) that " In 

 T. cajmt-serpentis there appears to be only two hepatic ducts, and both in it and in 

 TF. [Waldheimia] cranium the intestine is very short, terminating in a blind sac 

 before it reaches the ventral wall of the perivisceral chamber. It tapers gradually to 

 a point which is rounded, and suspended in its place by the mesentery." The genitalia 

 exhibit a somewhat different arrangement in T. caput-serpentis to what prevails in 

 W. Jiavescens ; " they are placed in large sinuses situated in the pallial lobes, one 

 at each side. These sinuses are, however, nothing more than the enlarged trunks of 

 the so-called pallial vessels or great pallial sinuses. They are four in number, two in 

 each lobe, and the genital band, which is placed within them, forms a thick convolute 

 layer, with small spaces between the folds." 



" In T. caput-se7'pentis the heart is more decidedly pyriform than in the other species, 

 and it is placed a little further back, the branchio-systemic vein passing for some distance 

 down the stomach beyond the central gastro-parietal band . . . The setse in T.capiU-serpeatis 

 are placed rather far apart from each other, and issue from the mantle at the points 

 corresponding to the marginal crenulations of the shell ; these crenulations give to the 

 pallial membrane a scalloped appearance. The setae are rather robust ; the marginal fold 

 is deep, and the follicules are of considerable length and rather wide, with their bases 

 su.rrounded with glandular matter, forming a roundish, red-coloured spot at the end of 

 each seta " {I. c. p. 829). The blood-system of the brachial ajiparatus has likewise been 

 well investigated by Hancock. He says : — " This is beautifully developed, and presents 

 considerable variety in the character of the several plexuses of which it is composed. The 

 walls of the great canal, the ridge supporting the cirri, the membranes that unite the 

 upper and lower members of the loop, that which connects the spirals, and those which 

 form the small canal or channel at the base of the cirri, as well as that forming the 

 sheath of the apophysary support, — all have their system of lacunes which inter- 

 communicate and compose the brachial system " [1. c. p. 831). 



Dr. Van Bemmelen questions the lacunary system as described by Hancock. He 

 concurs in the statement that in T. caput-serpentis and T. septentrionalis the sexes are 

 separate *. 



While treating of the perivisceral chamber, Hancock observes that in T. caput-serpentis 

 the four trunks may be recognized. " Here the trunks are fused so as to form on each 

 lobe two large, lateral, semilunar sinuses, in which the genitalia are placed. The external 

 margins of these sinuses give ofi" numerous, rather delicate branches, which dividing 

 dichotomously run to the pallial margin ; the branches next the middle line, which 



* •• Oil the Structure of the (Shells of liruchiopuds and Chitons," Ann. & Mag. Nat. Hist. 5th ser. vol. xi. p. 37y. 



