7^ 



NATURE 



[March 17, 192 1 



The eggs were found floating at a depth of 

 scarcely 100 metres from the surface ; they are 

 small, transparent spheres, i-3-i-4 mm. in dia- 

 meter. It will be noticed that the larva, albeit 

 clumsy to look at, nevertheless resembles an 

 ordinary fish larva, with the usual strong tail. 

 During the course of development, however, the 

 tail is soon reduced, while the dorsal and anal 

 fins, on the other hand, grow out strongly (see 

 Fig. 5, h). It is precisely this reduction of the tail 

 portion which gives the sun-fishes their remark- 

 able, as it were truncate, appearance, as seen in 

 Figs. I and 2. At a first glance it would appear 

 as if the third species [Mola lanceolata) had re- 

 tained the primary pointed tail (see Fig. 3). This 

 is, however, only apparently the case ; on study- 

 ing the development, it will be seen that the 

 primary larval tail here likewise soon disappears, 

 and that the pointed tail discernible in Fig. 3 is 

 a secondary formation. It almost seems, then, 

 as if Nature had repented of her own strange 

 whim, for scarcely has she deprived the species of 

 its tail when she replaces it with a new one ! 

 All three species, indeed, undergo striking altera- 

 tions in shape during development. When first 

 hatched, the length of the larva is considerably 

 greater than its height ; but the proportions are 

 soon reversed, and the height then exceeds the 

 length (Fig. 5, a and b). This state of things, 

 however, is not maintained ; at a length of barely 

 5-6 mm. the body of the oblong sun-fish {Ran- 

 zania iruncata) is already longer than it is high (in 

 the case of the Mola species this does not occur 

 until a far greater length is reached), and from 

 now onwards the height decreases in proportion 

 to the length until the final adult stage is attained 

 (compare Figs. 4, 5b, and 2, as well as Figs. 5, 

 c and a, and 3). 



At an early stage, so far back as the embryo 

 in the c^gg, we find the first indications of that 

 spinous equipment which is so characteristic a 

 feature of the sun-fish larvae and young. The 

 same spines can be recognised in both genera, 

 thus showing that these belong to the same type ; 

 otherwise, the development and size of the spines 

 differ widely, affording in this very feature a 

 means of distinguishing the three species with 

 the greatest ease. In the case of Ranzania trun- 

 cata the spinous equipment is comparatively 

 modest; in Mola lanceolata, on the other hand, 

 the spines attain such an enormous development 

 that at a certain stage they exceed the length of 

 the body. Five of the spines at this stage stand 

 out from among the rest in point of size, so much 

 so, indeed, as to deserve the name of horns. 

 Three of these are unpaired and set in the same 

 plane, directed forward, upward and down, the 

 remaining two being paired and set in a plane at 

 right angles to the first, and pointing obliquely 

 to the rear (Figs. 5, a, and 6). In all early stages 

 the two genera are easily distinguishable one from 

 the other by the structure of the bases of the 

 spines, which in Mola exhibit transverse ribs, 

 these being lacking in Ranzania. 

 NO. 2681, VOL. 107] 



The Mola larva; were invariably dead when 

 found in the net; those of Ranzania truncata, on 

 the other hand, I was now and then able to 

 observe in a living state. The upper portion of 

 the body (the entire part above the eyes) was 

 dark, while the lower glittered like silver. When 

 placed in a vessel full of sea-water, the larvae 

 could be seen shooting through the water at a 

 surprising speed, propelled by the extremely rapid 

 movements of the dorsal and anal fins, but appar- 

 ently with no good steering qualities. Fig, 5, 

 a and b, shows distinctly the two fins mentioned, 

 which are set in a manner resembling that of the 

 blades in a ship's propeller, here, however, always 

 placed vertically. 



The larvae were found in the open sea, not far 

 from the surface of the water : those of Mola 

 somewhat deeper than those of Ranzania. They 



Fig. 6. — Afo/a lanceo'ala, post-larval stage. Length, s mm. 

 Kront view. 



were very numerous in places, especially in the 

 Sargasso Sea, and we have found between one 

 and two hundred in the contents of a single net, 

 where they are difficult enough to discern 

 among the thousands of other small creatures. I 

 cannot, however, go further into the question of 

 distribution until we have been through the col- 

 lections thoroughly, which is a matter of con- 

 siderable time. 



In the literature of the subject, tiny larvae 

 of the sun-fishes have, so far as I am 

 aware, been mentioned and figured three 

 times : First, by Sir John Richardson (1844-48) 

 — this, strangely enough, only on account 

 of a drawing made by the botanist, Sir 

 J. D. Hooker, whq caught the specimen in a 

 tow-net in the South Atlantic; secondly, in 1898, 

 by the Danes Steenstrup and Liitken, from 

 material collected in the Atlantic by Danish sail- 



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