328 ME, E. E. WAITE OK THE EGG-CASES OP SHAEE:S. 



doubtless an individual peculiarity, indicates that the tendrils 

 are entire. 



The appendages with which the eggs of Sharks are furnished 

 serve to moor them in some suitable situation, otherwise they, 

 would be liable to be knocked about to the detriment of the 

 contained embryo, or even washed ashore, where their destruction 

 would be inevitable. The spiral appendages of G. Philippi are, 

 as has been shown, no exception to the rule ; the elastic ilanges 

 permit the egg to be forced further into a fissure, whence ex- 

 traction is resisted by the free edges of the ribbon catching 

 against the rock. 



Although, in a lesser degree, the egg-case of C. galeatus 

 possesses these spirals, they do not appear to have the same use ; 

 here attachment is effected by the entanglement of the tendrils 

 among seaweed. 



It may be of interest to inquire whether we are to regard the 

 spirals or the tendrils as the primitive appendages. Seeing that 

 C. galeatus possesses in its diminished spirals a useless appen- 

 dage, it may be inferred that such spirals are a bequest from 

 forms to whom they were serviceable. Also, since such a form 

 as G. Philippi having larger and serviceable spirals lacks the 

 tendrils, we infer that in G. galeatus the serviceable tendrils 

 are a later development, and that the spirals, now rudimentary in 

 function., are relics ; so the feature in common between such an 

 egg-case and those of the Dog-fishes appears to be a secondary 

 and independently acquired character. 



As before mentioned, very few theories have been advanced as 

 to the advantage of the peculiar form of the Cestraciont's egg. 

 An attractive explanation is offered by Mr. Grrant Allen in one 

 of his charmingly popular books *. His ingenious suggestion is 

 as follows : — 



" That w^ell-known frequenter of Australian harbours, the 

 Port Jackson Shark, lays a pear-shaped egg, with a sort of spiral 

 staircase of leathery ridges winding round it outside, Chinese- 

 pagoda wise, so that even if you bite it (I speak in the person of 

 a predaceous fish) it eludes your teeth, and goes dodging off 

 screw-fashion into the water beyond. There's no getting at 

 this evasive body anywhere ; when you think you have it, it 



* ' Science in Arcady,' p. 169. 



