626 SUMMARY OF CURRENT RESHARCHES RELATING TO 



derm of various individuals seem to be due to physiological relations. 

 The blastopore is in the form of a slit, the length of which is about 

 equal to that of the archenteron ; the hinder end of the gut never 

 separates from the ectoderm, and the anus arises at their point of 

 junction. Dr. Ziegler compares the mode of development which 

 obtains in Lamellibranchs with what is found in Gastropods, and 

 shows how they both point to a common primitive mode of develop- 

 ment. 



The trochophore of Cydas has all the organs homologous with 

 those found in the corresponding stage of marine Lamellibranchs and 

 Gastropods; as to the locomotor organs, the trochophore of Cydas 

 diverges somewhat from the marine Lamellibranchs and approaches 

 rather the Pulmonata. 



In describing the development of various organs, the author insists 

 that the pericardiac cavity is not, as has been thought, part of the 

 blood-vascular system, its fluid is not blood, and contains no blood- 

 corpuscles. The gill-lamella is, at first, a simple fold ; and, as differ- 

 entiation extends from before backwards, it is possible in one and the 

 same gill to observe various stages in the process of differentiation ; 

 from its lower margin the outer ectodermal layer gradually forms a 

 fold which has the form of a groove, and this gradually grows up- 

 wards ; there then appears on the lower margin of the fold a small 

 corresponding infolding of the inner ectodermal layer ; the lamellae 

 fuse, and vertical clefts appear in their substance. After describing 

 the appearance of the brood-pouches, the author concludes with a 

 short account of the genital glands ; these form at an early stage two 

 club-shaped masses which touch in the middle line ; the sexes are 

 united, and the disposition of parts is such that there appears to be 

 self-impregnation . 



Manner in which Lamellibranchs attach themselves to Foreign 

 Objects.* — Dr. J. T. Cattie describes the means by which the common 

 mussel attaches itself to foreign objects. When the foot commences 

 to grope about, it may become two or three times as long as the body 

 of the animal without finding any object within its vicinity ; it then 

 moves about till it finds some point of support ; when this is effected 

 there appears from the transverse cleft, which terminates the ventral 

 groove, a whitish substance which gradually becomes more opaque ; 

 sometimes the slit takes on the form of an equilateral triangle, and 

 then the quantity of matter which exudes from it is greater ; this 

 matter obviously comes from the cylindrical tubes which are scattered 

 in the glandular substance of the foot. A terminal plate having been 

 formed the foot is withdrawn, and the plate and the byssus are 

 merely connected by a delicate thread. The time necessary for an 

 animal of average size to form the plate varies between 55 and 90 

 seconds; in some cases two connecting threads become developed. 

 The terminal plate, when studied under the Microscope, was found 

 to be formed of thousands of small granules, irregularly distributed, 

 and varying considerably in size. The fine threads appear to be 



* Tijdschr. Nederl. Dierk. Vereen., vi. (1882-5) pp. 56-63. 



