BRACHIOPODA. 701 



heimia from the Trias, Argiope, Terebratulina, Macandreivia, Terebratella 

 and Megerlia from Jurassic times. Other living species belong to the 

 present epoch. There are a large number of extinct Palaeozoic forms, 

 some of which extend into the Mesozoic period ; and a relatively small 

 number of peculiar Mesozoic genera. 



A larval Brachiopod, supposed to be a Crania, has been described by F. Miiller 

 (Archiv f. Anat. und Physiol. 1860). It possesses an orbicular bivalve shell, the 

 dorsal larger than the ventral valve : the latter furnished with four pairs of stout 

 setae, the former with one pair, but also with a number of fine setae which bend 

 round the ventral valve. The mouth is overhung by a dorsal process or lip with 

 a singular resemblance to the epistome of Phylactolaematous Polyzoa, and is sur- 

 rounded by eight ciliated cirri : it is mounted on a contractile stalk. A pair of 

 eye-specks and of otocysts are situated, the latter dorsally, at the base of the 

 peduncle. There is a stomach. In swimming the oral stalk is extended, and the 

 cirri spread out ; in creeping the ventral valve is moved from side to side, the 

 fourth, a very long pair of ventral setae, together with the fine setae of the dorsal 

 valve, serving as fulcra. 



The larval Lingula is also free swimming. The two valves are at first orbicular. 

 The tips of the cirri are protrusible from the shell. The mouth, overhung by 

 a lip, lies in the centre of a circular lophophore turned towards the ventral aspect. 

 New cirri are added on each side of a median dorsal, or anterior cirrus. The 

 lophophore becomes gradually extended laterally. There is a pair of dorsal otocysts. 

 The peduncle arises as a hollow process at a comparatively late period. The 

 intestine is perhaps formed as an outgrowth of the stomach. The coelome grows 

 out into each mantle lobe, as two hollow processes. 



It is interesting to note (i) that in both these larvae the lophophore is free, and 

 at first orbicular, as in Argiope, though the conformation of the arms in the adult 

 is so different, and (2) that in Lingula the peduncle appears late. 



As to the Testicardines, Thecidium appears to have a delaminate gastrula; 

 Argiope, Terebratula and Terebratulina an invaginate. The coelome is formed by 

 a right and left diverticulum of the archenteron. In Argiope and Thecidium the 

 larva is affixed to the walls of the maternal brood-pouch by a filament attached to 

 its head. Argiope becomes divided first into two segments, then into three. The 

 most anterior acquires the shape of a ciliated umbrella with a short stalk, and a 

 row of especially long cilia at its margins. The second segment developes a dorsal 

 and ventral flap with ciliated edges, the future lobes of the mantle. The ventral 

 lobe carries four bundles of provisional setae. The third, a ciliated segment or a 

 small fourth (Shipley), becomes the future peduncle. Eye-specks four, rarely six 

 in number, are borne upon the head. The mantle-lobes and setae are eminently 

 mobile. The animal fixes itself by a secretion of the peduncular segment, and 

 then the two mantle-flaps are reversed over the head, the provisional setae lost, 

 the head segment becomes less and less prominent. The lophophore originates as 

 a nearly orbicular ridge on the inner aspect of the dorsal mantle-flap, and appears to 

 pass on the ventral side of the mouth, and the eye-specks lie near its ventral edge. 

 Finally, the valves of the shell are formed on the reversed mantle-lobes. The 

 larva of Thecidium is somewhat similar to that of Argiope, The head is not so 



