DEVELOPMENT OF TUB POLYZOA. 187 



lar interest. The free-swimming ciliated larva is provided 

 with a bivalve shell, and was originally described as a La- 

 mellibranch larva under the name of Cyphonautes. 

 Schneider discovered that it was a young Membranipora. 

 Barrois, Avho has traced its complete history, states that its 

 metamorphosis is fundamentally like that of the other ma- 

 rine Polyzoa. Flustrella Idspida passes through a similar 

 Cyphonautes stage. 



In Loxosoma young resembling the adult bud out like 

 polyps. Nitsche does not regard this budding process as an 

 alternation of generations, but states that in Polyzoa of the 

 family of Vesiculariidce, this may occur, as in the latter 

 some cystids form the stem, and others (the zocecia] produce 

 the eggs. Most fresh-water Polyzoa reproduce by the devel- 

 opment of winter buds or eggs surrounded by a horny case, 

 and developing from the funiculus. 



To recapitulate : the Polyzoa increase (a) by budding ; (b) 

 by normal (summer) eggs, and by producing statollasts, or 

 winter eggs. In reproducing from summer eggs, the young 

 pass successively through a morula, blastula, yastntla and 

 trochosphere stage before attaining maturity. 



The most aberrant Polyzoan is Rliabdopleura mimbilis Sars, 

 v.hich occurs in from 100 to 300 fathoms on the coast of 

 Norway. It differs from other forms by the want of an en- 

 docyst or mantle, whence it moves up and down in its cell, 

 without being attached to the opening, the muscles usually 

 present being wanting, the cord by which it is attached to 

 the bottom of its long, slender tubular cell being contractile. 

 The lophophore is much like that of the fresh-water Poly- 

 zoans, consisting of two long arms, bearing two rows of 

 slender tentacles. The cpistome is represented by a large 

 round disk. 



The marine Polyzoa occur at great depths, and a few species 

 are cosmopolitan ; the type is very persistent, and occurs 

 in the oldest Silurian strata, the earliest forms being very 

 similar to their living descendants. 



