PALLIAL FUSION AND PROLONGATIONS. 
199 
and out-flowing streams, as this fusion almost invariably takes place 
near the middle of the posterior margin and cuts off fi’om the great 
ventral cleft a small exhalent postero-dorsal orifice, directly opposite 
to the anus ; the species with this single point of fusion, which thus 
divides the gTeat mantle opening into two apertures, a large antero- 
veutral pedo-hranchial cleft and a smaller dorso-posterior anal opening, 
are termed Biforate. The Naiads illustrate this stage of specialization, 
although the anal cleft is divided by a central subsidiary fusion into 
two distinct though coiiiiected orifices. 
The succeeding stage is characterized by a second point of fusion 
of the mantle margin, which invariably takes place at a point more 
ventral than the first and near the junction of the ventral and posterior 
margins, separating the great pedo-hranchial cleft into a branchial and 
a pedal opening, the anterior one serving for the protrusion of the 
foot, and the posterior one, which adjoins the anal pallial opening, 
forming the branchial or inhalent aperture ; there are thus three 
openings, which subserve three distinct functions ; such forms are 
kuowm as Triforate. 
This process of fusion is carried to a still further extent in species 
not within the scope of our studies, both by more points of con- 
crescence and by their gTeater extent. 
The Prolongation of the respiratory apertures to form retractile 
siphons is a further and extreme specialization of the fusion of the 
mantle margins, and appears to be a characteristic of an aquatic 
existence, being strikingly developed among the carnivorous marine 
Streptoneures. Amongst our species this feature is not noticealily 
developed, although in Vivipara the i-ight cephalic lobe, which is an 
outgrowth of the foot, becomes folded and forms a siphon which con- 
ducts the respiratory water to the branchial cavity. 
Fig. 393. — Limtuea stagnalis V7\.x. fragilis 
(L.), Chislehurst, Kent, collected by Mr. S. C. 
Cockerell, showing the respiratory aperture 
contracted. 
Fig. 394. — Li))i>upa pcregc>'(^ (Miill.), Christ- 
church, Hants., showing the prolongation of 
the pulmonary aperture as a respiratory tube. 
Amongst the Basommatophora the Limnwcc especially are capable 
