366 



MOLLUSCOIDEA 



PHYLUM V 



(Fig. 537, Ä, B) ; (2) the mesemhryo, or blastosphere, a multi-segmented larva with an 

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

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

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

 Segment carrying four biindles 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 lobes (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 



Ärgyrotheca neapolitana (Schacchi). A, Neoembryo ; completed 

 cephalula 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 Central sides of body ; vp, Muscles from ventral side of body to 

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



Fig. 540. 



ArgyrothecaneapoUtana(ßchsiCch.i). 

 A, Phylembryo ; Brachiopod show- 

 ing Shell (protegulum), beginning of 

 tentacles of lophophore (/), obsoles- 

 cence of eye-spots, and formation 

 of Oesophagus ; t, Hinge-teeth ; vp, 

 Ventral pedicle muscles. B, Nepio- 

 nic Brachiopod, showing distinct 

 tentacles of lophophore, mouth and 

 stomach, and transformation of 

 muscleg from typembryo ; ad, Ad- 

 ductors ; di, Divaricators ; vjj, 

 Ventral pedicle muscles (after 

 Kovalevski, 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, orsixth stage ofdevelopment, the embryonic shell, ov protegulum, 



is completed; the tentacular lobes of 

 the lophophore, or brachia, appear ; 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 



Fig. 541. 



Thecidea (Lacazeiia) mediterranea (Risso). Recent. Beecher in many genera representiuff 



A, Dorso- ventral longitudinal section of cephalula; ..r,r,T.Ur oll fl,^ l^o^- -p -t a ^-i 



h, Head; d, Dorsal mantle lobe; v, Ventral mantle ^l^^rly all the leadmg families of the 



lobe; ds, Beginning of dorsal ya.ive; dei, Shell piate class, and therefore it may be inferred 



formmg on dorsal side of body; p, Pedicle. B, Dorso- 4.1, + +t. , . *^ xni-ciicu 



ventral longitudinal section of typembryo ; vs. Ventral '^"-^^ tue protegulum IS common tO all 



valve; hl, Hinge-line of dorsal valve. C, Adult speci- BmchioDods Tt 1=1 <iPminh^nM'\or> ^1. o^rr.; 



men seen from the dorsal side, showing ventral area and -^^f ^^^lopo^S. It IS SemiCllCUlar or semi- 



deltidium. (^ and iJ after Kovalevski; C after Beecher.) elliptical in Outline, with a straight or 



arcuate hinge-line, and no cardinal area. 



The prototype preserving throughout its development the main features of the 



