CHANGES IN THE SHAPE OF THE SHELL. 75 



of a ring surrounding the oesophagus ; in this ring are found a 

 ventral ganglionic thickening, two lateral ganglia and two dorsal 

 otocysts. In Franz Muller's larva (Fig. 38), paired eye-spots (a) 

 and auditory vesicles (o) were also observed. These sensory organs 

 degenerate in the course of further development. 



The rudiment of the peduncle now arises at the posterior end of 

 the body, and soon grows to a considerable length; by means of this 

 the attachment of the larva takes place. In the further course of 

 metamorphosis the lophophore assumes its final shape ; characteristic 

 changes occur in the form of the shell-valves, and the mantle-sinuses 

 arise as horn-shaiied diverticula growing out from the l:)ody-cavity, 

 while the rows of setae found in the adult appear on the margin of 

 the mantle. 



III. Changes in the shape of the Shell. 



Certain transformations undergone by the shell of Terehratulina 

 in the course of ontogenetic development were pointed oiit by Morse, 

 who drew attention to the fact that the shell in this form, during its 

 development, passes through stages in which it resembles the shells 

 of AlegerUa or Argiope (cf. p. 73). These developmental changes of 

 the shell have recently been investigated by Beecher (No. 1 ), 

 who proposes to name the first rudiment of the shell of a newly 

 metamorphosed larva, Avhieh consists of a horny, chitinous, or 

 cuticular secretion, the protegulu7n. The most primitive and the 

 most widely distributed form of protegulum is a semi-circular or 

 semi-elliptical plate (Fig. 39, 2^) with a straight hinge line which, in 

 length, corresponds to the greatest breadth of the plate. The shape 

 of the protegulum may be modified in individual cases, the future 

 form of the shell then exercising an influence. Only in rare cases 

 does the shell, after increasing in size by the formation of new 

 layers, retain the original shape of the protegulum, the zones of 

 growth here running j^arallel with the periphery. An example 

 of this is afi'orded by Paterina (Oholus) Jahradorica. Other forms 

 {e.g., Orhiculoidea) pass through a Paferma-like stage in their 

 ontogeny (Fig. 39 A). 



The phases through which the shell-valves pass in later stages 

 are connected with the relative length of the i^eduncle, with the 

 angle formed by the longitudinal axis of the animal and the 

 substratum, and Avith the consequent greater or slighter mobility 

 of the body. The manner of insertion of the peduncle is also of 

 significance in connection with the transformations in the form 



