ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 869 



Mollusc a. 



Operculum, and Foot-glands of Gastropoda.*— F. Houssay finds 

 it necessary to distiuguisli tlie suprapedal from the other glands of 

 the foot, on account of the jjosition of their orifice, their own position, 

 and the great complexity of structure to which they may attain. 

 The rest, though less highly developed, are more interesting from the 

 point of view of homology; they are typically formed of three 

 portions, the transverse groove, the median canal, and the folded 

 cavity, which opens on the middle of the ventral surface of the foot. 

 By their structure, and sometimes also by their function, they are to 

 be compared to the byssus-glands of the Lamellibranchiata ; although 

 in the latter the glands are better developed, yet they have no im- 

 portant additional parts. In the Lamellibranch, the glandular appa- 

 ratus of the foot is essentially made up of a longitudinal ventral 

 groove, into which there opens anteriorly a longitudinal canal which 

 may be branched, and which stops before it reaches the byssal cavity. 

 The interior of the foot contains a well-developed gland which un- 

 interruptedly surrounds the small longitudinal canal, and the folded 

 cavity. The byssus-gland appears to be formed by the union of the 

 glands which surround the two organs, and this union may be regarded 

 as due to the great development of these parts. 



The author suggests as objections (1) that the longitudinal canal 

 of the Gastropoda opens into the most anterior part of the foot, while 

 in the Lamellibranchiata it opens much lower down : (2) that the 

 canal is branched in Lamellibranchs, but simple in Gastropods : (3) 

 that the secretions of the glands are too difierent to be comparable 

 one with the other ; and (4) that the transverse groove of the foot of 

 Gastropods is suppressed and a longitudinal groove added on, in the 

 Lamellibranch. 



To these objections there seem to be the following answers :— (1) 

 The long canal of Gastropods has an upper lip of a certain thickness ; 

 it is probable that, in Lamellibranchs, this lip has become consider- 

 ably developed, or has formed the anterior portion of their foot. It 

 is now known that this region is very generally formed of a mass of 

 mucous glands, which are analogous, if not identical, with those which 

 surround tlie transverse groove in Gastropods ; these cells have not 

 lost their muciparous fimction and they take no part in the formation 

 of the byssus. (2) The reason why the canal is branched in one 

 case and not in the other, is to be found in the fact that one has a 

 gland much better developed than the other. (3) The difference in 

 the secretions is no real objection ; there is no greater difference 

 between glands which secrete mucus or chitin, and those that pro- 

 duce chitinous or calcareous matter ; and the latter obtains in the 

 case of the byssus-apparatus. In some cases, indeed ( Venus decussata) 

 the byssus-gland does produce mucus. (4) There is a small trans- 

 verse groove, or the representative of it, in the foot of Lamelli- 

 branchs, and the addition of a longitudinal groove is to be correlated 

 with the greater development of the gland and its new function. 



* Arch. Zool. ExfK-T. ot Glen., ii. (1884) pp. 171-2S8 (8 pis.). 



