350 PALAEOSTRACA. 



eyes and the nerves connected with them, while the third pair, in Scorpio, yields 

 the optic ganglia of the lateral eyes, but, in Limulus, is related to a small 

 sensory organ (Fig. 162, s). The lateral eyes belong to the third (Patten), 

 fifth (Kingsley), or, according to other authors, the fourth thoracic segment, 

 and thus represent post-cephalic structures. Each optic ganglion is. continued 

 posteriorly into a nerve strand, a kind of lateral nerve running along the outer 

 side of the limb-rudiments, and connected with a sensory organ in each 

 segment (Fig. 162, so x -so e ). According to Patten, these sensory organs (with 

 the exception of the rudiments of the lateral eyes) have usually only a temporary 

 importance, and soon disappear. Kingsley, on the contrary, holds that the 

 first pair (sox) yields the median eyes, the second a peculiar and as yet 

 undescribed sensory organ, while the third disappears ; from the fourth is 

 developed the dorsal organ (so 4 ) of Watase, which persists for a long time ; the 

 fifth passes into the compound lateral eyes, and the sixth, finalty, degenerates. 

 Until these statements have been confirmed by more detailed observations, they 

 must be regarded Avith some scepticism.* We must refer the reader to Patten's 

 treatise cited above for details as to the development of the brain in Limulus, 

 his account of which, in the absence of satisfactory figures, is hardly com- 

 prehensible. We are also unable, on account of the fragmentary character of 

 the statements made on the subject, to decide how far Packard's more recent 

 researches (No. 27) as to the structure of the brain in Limulus can be brought 

 into agreement with Patten's views. Packard emphasises the fact that the 

 cheliceral ganglion in Limulus does not fuse with the brain, but remains distinct. 

 The brain proper sends off only the nerves to the median and lateral eyes, and 

 two pairs of integumental nerves (frontal and inferior frontal nerves). It 

 consists of three pairs of lobes : those of the lateral eyes, the median eyes, and 

 the cerebral lobes proper. In the absence of figures, it is impossible to obtain a 

 clear idea of the true relationships of these cerebral lobes. 



Patten considers that the development of the median eyes in 

 Limulus closely resembles the processes observed in Scorpio. Here, 

 as in the latter, there are two (or perhaps, according to Patten, four) 

 invaginations which, shifting backward, unite in the median line to 

 form a sac with a common posterior aperture. This sac gives off 

 anteriorly two tubular processes, the blind ends of which, becoming 

 apposed to the hypodermis of the cephalo-thorax, are transformed 

 into the median eyes, while the parts of the processes that remain 

 change into the optic nerves. 



The development of the lateral eyes, now made more fully known 

 by Watase (No. 30), is much simpler. We can trace the lateral 

 eyes back to a highly -differentiated part of the ectoderm (hypo- 

 dermis). The actual rudiment of the compound lateral eye 

 (Fig. 163, la) is a thickened point of the ectoderm (hypodermis) 

 near the so-called dorsal organ (do), which apparently belongs to 



* [Kingsley, in his latest work, withdraws his account of the segmental 

 sense organs, and would now regard those structures as glandular. KlSHlNOUYB 

 (No. 111.) funis four parts in the developing brain, and regards the lateral i 

 phalio, not thoracic. — Ed.] 



