366 



MOLLUSCOIDEA 



PHYLUM V 



(Fig. 537, A, B) ; (2) tlie mesemhryo, or blastospliere, a multi-segmented larva with an 

 internal cavity (Fig. 537, C); (3) the metcmhryo, or gastrula stage (Fig. 537, D); (4) 

 the neoembryo, or the ciliated Cephalula stage, which consists atj first of a cephalic lobe, 

 bearing eyes in Argyrotheca, and a caudal lobe, to which is added later a thoracic 

 segment carrying four bundles of setae, while at the same time the dorsal and ventral 

 sides of the latter segment become extended over the caudal lobe, and are progres- 

 sively defined as two lohes (Fig. 538) ; (5) the typemhryo, or larval stage, in which 

 the dorsal and ventral thoracic lobes, or mantle, fold over and enclose the cephalic 

 lobe (Fig. 539, B). Upon the mantle lobes, either 

 before or after turning, there is a corneous integument 

 which develops into the protegulum before the for- 

 mation of the true shell. The caudal segment be- 

 comes the pedicle, and may in this stage serve to 

 attach the larvae to foreign objects, or the pedicle 



Fig. 539. 



Arfiyrotheca neapolUana (Scbacchi). A, Neoembryo ; completed 

 cejihalula stage. B, Typembryo ; transformed larva resulting from 

 folding upwards of mantle lobes over cephalic segment ; ad, Muscles 

 from bundles of setae to sides of body cavity ; di, Muscles from dorsal 

 to ventral sides of body ; rp. Muscles from ventral side of body to 

 caudal segment or pedicle (after Kovalevski, from Beecher). 



Fig. 540. 



ArgyrnthecaneapoHtana(Schacchi). 

 A, Phylembryo ; Brachiopod show- 

 ing shell (protegulum), beginning of 

 tentacles of lophophore (1), obsoles- 

 cence of eye -spots, and formation 

 of oesophagus ; t, Hinge-teeth ; vp, 

 Ventral pedicle muscles. B, Nepio- 

 nic Brachiopod, showing distinct 

 tentacles of lophojjhore, mouth and 

 stomach, and transformation of 

 muscles from typembryo ; ad, Ad- 

 ductors ; di, Dlvarleators ; vp, 

 Ventral pedicle muscles (after 

 Ko\alevski, from Beecher). 



may remain undeveloped for a time. A rudimentary digestive tract is present, and 

 also four pairs of muscles, which later become the adductor, diductor and ventral 

 pedicle muscles. 



In the phylembryo, or sixth stage of development, the embryonic shell, ov protegulum, 



is completed ; the tentacular lobes of 

 the lophophore, or brachia, api^ear ; the 

 four bundles of setae are dehisced ; ob- 

 solescence of the eyes occurs, as well as the 

 agreement of the muscular system with 

 that in adult forms (Fig. 540). 



The protegulum has been observed by 

 Beecher in many genera, representing 



Fio. 541. 

 Thecidea (iMcazella) mcditerranea (Risso). 



Recent. . ^ 



A, Dorso-ventral longitudinal section of cephalula; nom.iir nil +!-.« l^n,lT„„ e ■^■'' c i.^y 



7i, Head; d, Dorsal mantle lobe; ,-, Ventral mantl^ ""^^^^^ ^^^ ^^^ leading families of the 



lobe ; rfs, Beginning of dorsal vahe ; del, Shell plate class, and therefore it may be inferred 



forming on dorsal side of body ; p, Pedicle. B, Dorso- fi i. ii j. i • ti 

 ventral longitudinal section of typembryo ; vs, Ventral ^^^''^^ ''"^ protegUlUlll IS Common tO all 

 valve; /iZ, Hinge-line of dor.sal valve. C, Adult speci- Brachiopods. It is Semicircular or semi- 

 men seen from the dorsal side, showing ventral area and ,,. . 1 . 

 deltidium. (.4 and « after Kovalevski; C after Beecher.) eliiptical in outlmo, with a straight or 



arcuate hinge-line, and no cardinal area. 



The prototype preserving throughout its development the main features of the 



