Καν λλ 
νὰ 
ON FRESHWATER POLYZOA. 321 
stance, and internally it presents a rough uneven surface. The two tunics 
of the gemma are to become the ectocyst and endocyst of the future cell. 
By this time the gemma has become considerably elongated and has 
acquired a clavate form, and its cavity begins to be cut off from that of 
the parent cell by the formation of a septum. We next perceive that a 
rounded mass has formed in the substance of the lining tunic, near the wide 
extremity of the gemma, and projects into the interior of the latter. In this 
mass we soon perceive a cavity, which, when viewed in front, appears sur- 
rounded by a slightly waved oval ring which is afterwards to become the 
tentacular crown of the polypide. The ring is at first quite simple, re- 
sembling a mere fold of thickish membrane, but in a short time it presents 
all round a series of minute tubercles, the rudiments of the future tenta- 
cula. Delicate fibres may now be distinctly seen passing from the little 
mass in which these appearances have been presenting themselves to the 
walls of the cavity of the gemma; these fibres are the rudimental retrac- 
tors of the polypide. Circular fibres may also be now seen in the lining 
membrane of the gemma; these are chiefly collected near its proximal end, 
and are to become the parietal muscles of the adult. The tentacular sheath 
may about the same time be distinctly seen extending from the base of the 
rudimental tentacula to the walls of the cavity in which the young polypide 
is suspended, and fibres which are to become the superior parieto-vaginal 
muscles may be observed in connection with it. The rudimental polypide has 
now become somewhat enlarged below the tentacular ring, and here presents 
in its interior a cavity. This cavity is at first simple and continuous, but as 
the inferior extremity of the polypide continues to elongate, we soon find it 
divided into three distinct regions, which are the first indication of cesophagus, 
stomach and intestine. By the elongation of the tentacles, the tentacular 
crown has now acquired nearly its full development. 
Up to this period the young polypide has been entirely shut off from all 
communication with the external water, and its nutrition must have been 
effected through the general nutrition of the colony ; now, however, an opening 
occurs in the gemma just over the tentacular crown, and the last stage of de- . 
velopment is entered on. The tentacular crown rapidly acquires its complete 
form, the inferior extremity of the alimentary canal becomes elongated into 
the great cul-de-sac of the stomach, the muscles are by this time all formed, 
and the polypide is capable of exsertion and retraction. It is now no longer 
dependent for its growth on the general nutrition of the colony, but has be- 
come an independent being, obtaining its food from without, and submitting 
it to the action of its own digestive system. κ᾿ 
From the description here given of the development of the gemma in Pa- 
Iudicella, we must be at once struck with its remarkable similarity to that of 
the gemma in Zaguncula, as elaborately described and figured by Van Be- 
neden*; a glance indeed at, the figures in Van Beneden’s memoir is sufficient 
to convince us how closely the freshwater genus resembles that of the sea in 
the interesting process whose details we have been just following. 
2. Reproduction by Ova.—All the freshwater Polyzoa produce true ova, 
which are formed in a definite organ or ovary. From the existence of a true 
ovary and ova, we are at once led to expect the co-existence of a male organ. 
That a testis is present in all the species of freshwater Polyzoa, there can I 
think now be little doubt. In most of the genera I have met with an organ 
which I have little hesitation in viewing as a testis, though, with the exception 
of Paludicella, the demonstration of such an organ is somewhat obscure. In 
this genus, however, I have had the most satisfactory demonstration of both 
* Loe, cit, 
1850. : ε 
