262 SYNOPTIC COLLECTION. 



free (No. 644). The two halves of the sac then reunite 

 and the sac thereby formed becomes rilled with spermato- 

 zoa. The arm with the sperm is detached and finds its 

 way to the mantle cavity of the female. No. 645 shows 

 the two large arms of the female that have become greatly 

 modified in structure. In the alcoholic specimen these 

 are much contracted. They are usually applied closely to 

 the outer surface of the shell and are not carried as sails 

 though often so figured in the books. 



The Argonaut propels itself through the water in pre- 

 cisely the same way as the squid, having the same 

 hydraulic apparatus. 



The Mollusca constitute a subkingdom of immense size 

 and unnumbered variations. Underlying these variations, 

 however, there is a fundamental unity. Most Mollusca 

 pass through a trochophore and a veliger stage in their 

 development ; most possess a shell gland the first product 

 of which is a simple, unornamented, colorless or slightly 

 colored, plate-like shell. If this plate-like shell divides 

 into two parts the result is a bivalve shell, the distinctive 

 character of the class of Pelecypods. If it becomes a 

 spiral cone, the univalve shell and the class of Gastropods 

 results. If the spiral cone becomes chambered, the 

 product is the shell peculiar to the Cephalopods. 



Free-moving, sedentary, and boring habits produce 

 modifications in the shell. Among Pelecypods the free- 

 moving species, as a rule, have equivalved. symmetrical 

 shells, while the sedentary forms are unequivalved and 

 asymmetrical. The boring habit in both Pelecypods and 

 Gastropods tends towards the reduction of the shell, until 

 only a vestige of it remains. Other causes have produced 

 a similar reduction. The free-swimming habits of the 

 Heteropods have resulted in the development of a fin-like 

 organ, while the shell, being of little use, has become 

 small and inconspicuous. The Nudibranchs carry the 

 process still farther, since they possess a shell in the 



