674 



SYSTEMATIC niSTORY OF THE rNFFSORIA. 



freely out of the ovary when the latter 

 i3 ruptured. The nuclei situated nearest 

 the centre of the ovary appear to be 

 successively selected for development. 

 One of these nearest the surface attracts 

 around itself a small portion of the gra- 

 nular protoplasm^ detaching it from the 

 remaining contents of the organ, though 

 in close contact with them. The portion 

 thus specially isolated gTadually enlarges, 

 assuming at the same time a darker hue, 

 whilst, fi'om its central position, it par- 

 tially divides the upper from the lower 

 half of the remaining ovarian protoplasm. 

 At the same time the central nucleus 

 sometimes undergoes some slight en- 

 largement, and its nucleolus appears to 

 become absorbed. The position of this 

 nucleus in the centre of the ovum is 

 now indicated by an ill-defined trans- 

 parent spot ; but on bursting the proto- 

 plasmic mass, it is seen to be a small 

 spherical cell about 1-1000" in diameter, 

 having ver}^ thin pellucid walls and 

 scarcely any visible cell-contents. When 

 the ovum thus segmented from the ova- 

 rian protoplasm has attained its full size 

 (xxxvii. 17 o), it becomes invested by a 

 thin shell, which is apparently a secre- 

 tion from its own surface." 



" The ovum being now ready for ex- 

 pulsion, it is slowly forced down to the 

 lower part of the ovary, the stomachs 

 being drawn upwards and to one side in 

 order to make way for it. Yielding to 

 the pressure produced by the successive 

 contractions of the body, the ovum 

 sweeps round the inferior border of the 

 lower stomach, and, passing through the 

 dilated oviduct, enters the cloaca. The 

 latter canals become entirely everted, as 

 is the case when the excrements are dis- 

 charged ; and by a sudden contraction 

 the ovum is expelled." 



Professor Williamson minutely de- 

 scribes the conversion of the yelk into 

 an embryo — the successive segmenta- 

 tions of the nucleus and surrounding 

 yelk, imtil the whole becomes a cellular 

 mass, as in Lacimdaria. The first visible 

 evidence of life is the production of a 

 few moving cilia, especially near the 

 future head, followed first by ti'aces of 

 the dental apparatus, then by the de- 

 velopment of the various organs, in- 

 cluding the two eye-spots, soon after 

 which the young animal escapes from 

 its shell. 



" Almost immediately after its escape 



Genus FLOSCULARIA (XXXII. 384, 385 ; and woodcuts).— These crea- 

 tures possess when young two eye-spots. Several lobes surround the head, 



from the e^^, the young Melicerta 

 stretches itself out, and, everting the 

 anterior part of its body, unfolds several 

 small projecting mamillte (xxxvii. 16), 

 covered with large cilia, by means of 

 which it floats freely away. The ciliated 

 mamiUse at this stage of growth are not 

 unlike those seen in Notommata clavu- 

 lata, but they soon enlarge and become 

 developed into the fiabelliform wheel- 

 organs of the matured animal." In this 

 stage all the organs of the perfect animal 

 are present, showing that the creature 

 passes through no larval form, and that it 

 is not identical with the Ptygiira, as Ehr- 

 enberg and others have thought. After 

 swimming about some time, a dark- 

 brown spot disappears from the posterior 

 part of the body, followed by the eye- 

 specks, when, the same writer adds, " the 

 animal attaches itself by the tail to some 

 fixed support, anddevelopes from the skin 

 of the posterior portion of its body a thin 

 hyaline cylinder, the dilated extremity 

 of which is attached to the supporting 

 object. This structure has already been 

 noticed b}' Dr. ManteU ( TJioughts on Ani- 

 malcules), though I have never seen it 

 so largely developed as is represented in 

 his figures. The yoimg animal, ha^dng 

 chosen a permanent resting-place, com- 

 mences the formation of its singular 

 investing case. I have verified Dr. Man- 

 tell's account of the position occupied 

 by the first-formed spheres. They are 

 arranged in a ring round the middle of 

 the body (xxx^t:i. 15), and are for some 

 time imattached to the leaf or stem 

 which supports the animal. They appear 

 to have some internal connexion with 

 the thin membranous cylinder. At first 

 new additions are made to both extre- 

 mities of the enlarging ring; but the 

 jerking constrictions of the animal at 

 length force the caudal end of the cylin- 

 der down upon the leaf, to which it 

 becomes secui-ely cemented by the same 

 viscous secretion as causes the little 

 spheres to cohere." "When the ova are 

 discharged fi'om the cloaca, they succes- 

 sively fall into the cavity of the tessel- 

 lated case, where they undergo develop- 

 ment. I have often foimd as many as 

 foui- in one case in various stages of 

 progress. It is whilst the eggs are thus 

 protected that the young animals burst 

 their shells, s^^^mming out at the fi'ee 

 extremity of the case as soon as they 

 are liberated." 



