ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 417 
In Hydractinia echinata then the period consists of three terms—poly- 
pite—gonoblastidium—sporosac. 
A still further advance in complication is afforded by those forms in which 
a gonoblastocheme is developed, as in many of the Campanularide—Cam- 
panularia Johnstoni for example. Here we have polypite and gonoblastidium 
in the same order as in Hydractinia ; but the gonoblastidium, instead of giving 
origin directly to sexual zooids, developes a new kind of zooid, as a bud from 
its sides, in the form of a non-sexual medusa or gonoblastocheme ; and it is 
upon this gonoblastocheme that devolves the function of producing, by a 
process of gemmation, the ultimate sexual zooid in the form of a sporosac. 
In Campanularia Johnstoni, therefore, the period will consist of four 
terms—polypite—gonoblastidium—blastocheme—sporosac. 
It will be seen that, in the cases enumerated above, we have examples 
of three different types of alternation—the binary, the ternary, and the qua- 
ternary, this last presenting the highest order of complication which we know 
of among the Hyprorpa. 
The three types may be conveniently formulated as follows :— 
hes EO eR Se a ee ere eee 
More.. b bet, a Oe * 5. TS eee eee 
Quaternary. Ternary. Binary 
eS 
an aeG eda! GF cd Seer ener ser COL» 
Here every successive period is represented in the first series by two 
terms, in the second by three, and in the third by four; the “ period” in 
each case repeating itself indefinitely like a circulating decimal, so as to 
represent in this repetition the indefinitely extended life of the species, while 
the life of the individual is expressed by each “ period” singly. 
It will be noted that the ‘“‘ terms” which, in the above formule, combine 
to form a period may each consist of many zooids. Itis, in fact, very rare to 
find any term consisting of but one zooid. As an example of this latter 
condition, Corymorpha nutans may be adduced. In this hydroid, whose 
life-series belongs to the binary type, the polypite term,ada'a',... &¢., 
consists of a single zooid, while the gonophore term, 6 b' b”, . . . &e., may 
consist of a vast number of zooids,—the single polypite giving origin to 
“numerous gonophores in the form of free medusa-buds. In almost every 
other case, however, the simple polypite becomes, by gemmation, a com- 
" posite trophosome. 
Direct Development of the Medusa from the Egg. Metamorphosis.—A 
question, however, of great importance still remains for discussion; for we 
have yet to determine whether the phenomenon of the so-called alternation 
.of generations is a universal fact among the Hyprorpa, presented by every 
Species in the course of its development. 
In by far the majority of cases in which the development has been suc- 
cessfully traced, the life-series of the individual has presented a polypoid 
1863, 25 
