302 



MOLLUSCOIDEA 



SUB-KINGDOM V 



becomes the pedicle, and may in this stage serve to attach the larvae to foreign objects, 

 or the pedicle 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 the embryonic shell, or protegulum, is completed ; the tentacular 

 lobes of the lophophore, or brachia, appear ; the four bundles of setae are dehisced ; 

 obsolescence of the eyes occurs, as well as the agree- 

 ment of the muscular system with that in adult forms. 



The protegulum has been observed by Beecher in 

 many genera, representing nearly all the leading 

 families of the class, and therefore it may be inferred 

 that the protegulum is common to all Brachiopods. 

 It is semi-circular or semi-elliptical in outline, with a 



FIG. 502. 



Cistella Neapolitana, Schacchi. A, Neoembryo ; completed cepha- 

 lula 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 ; vp, Muscles from ventral side of body to caudal 

 segment or pedicle (after Kovalevski, from Beecher). 



FIG. 503. 



Cistella Neapolitana, Schacchi. 

 A, Phylembryo; Brachiopod show- 

 ing shell (protegulum), beginning 

 of tentacles of lophophore (I), ob- 

 solescence of eye-spots, and forma- 

 tion of oesophagus ; t, Hinge-teeth ; 

 vp, Ventral pedicle muscles. B, 

 Nepionic Brachiopod, showing 

 distinct tentacles of lophophore, 

 mouth and stomach, and trans- 

 formation of muscles from typern- 

 bryo ; od, Adductors ; di, Divari- 

 cators ; vp, Ventral pedicle muscles 

 (after Kovalevski, from Beecher). 



straight or arcuate hinge -line, and no cardinal area. The prototype preserving 

 throughout its development the main features of the protegulum, and showing no 



separate or distinct stages of growth, is 

 represented by the genus Paterina (Fig. 

 505). 



So far as observed, the protegulum, 

 or Paterina stage, in the Atremata and 

 Telotremata is followed by the Obolella 

 stage of nearly circular outline. After 

 Recent, this stage, specific characters appear, and 



Telotremata 

 is, Beginning of dorsal valve ; 'del, Shell plate forming there are usually developed the first rudi- 



iide of body ; p, Pedicle. B, Dor 



FlG 504 



Thecidia (Lacazella) Mediterranea, Risso. 



ADowo- ventral longitudinal section of cephaiuia ; i n the open delthvrium of the 

 h, Head ; d, Dorsal mantle lobe ; v, Ventral mantle lobe ; . . ". , , ^ 



on dorsal side of body ; p, Pedicle. B, Dorso-ventral m ori f a ^f fU /lolHYIial r>lafp<a Tn flip 



longitudinal section of typembryo ; vs, Ventral valve ; ments ot tne deltldial plates. In t 



hi, Hinge-line of dorsal valve. C, Adult specimen seen Protremata, the Paterina stage is not 



from the dorsal side, showing ventral area and deltidium. . n TT , , ,, p., , 11o .,' r.-. f +1,,. 



(A and B after Kovalevski ; C after Beecher.) followed by the Obolella Stage, but tlie 



wide delthyrium of the protegulum is at 



once affected and modified by the prodeltidium, which develops into the deltidium. 

 In the Acrotretacea, belonging to the Neotremata, there is often developed a true 

 deltidium, showing the close relationship of the order with the Protremata, and the 

 dissimilarity of these two orders to Atremata and Telotremata. In the Discinacea, 

 belonging to the Neotremata, the pedicle opening is an open notch in the posterior 



