MUSEUM OF COMPARATIVE ZOOLOGY. 65 



Tatten's statements is not so serious as might at first be supposed, and 

 that, admitting the relations already mentioned between the two plans 

 of arrangement, the observations of these two writers can be explained 

 without supposing either of them to be wrong. 



The probable method of rearrangement by which the hexagonal plan 

 is converted into the tetragonal has been suggested in a previous paper 

 (Parker, '90 a , p. 50). It involves two changes: the conversion of the 

 hexagonal outline of the ommatidium, as seen in the corneal facet, into 

 a square one, and the slipping of the rows of ommatidia one on the other, 

 so that the lines which bound the four sides of each facet finally form 

 parts of two series of lines which cross each other at right angles. 



A condition somewhat intermediate between the hexagonal and tetrag- 

 onal arrangement is shown in the retina of Crangon (Plate X. Fig. 123). 

 In this genus the outlines of the ommatidia as seen in the facets are 

 square, although their arrangement suggests the hexagonal type. The 

 permanent grouping of the ommatidia in Crangon represents a stage 

 slightly in advance of the condition seen in some young lobsters (com- 

 pare Parker, '90% Plate IV. Fig. 55), and the particular features in 

 which this advance is shown are two. First, the distal retinular nuclei 

 in Crangon (Fig. 123) are grouped in pairs, more as they are in adult 

 lobsters, and not in circles of six, as in young ones (compare Parker, 

 '90 a , Figs. 5 and 55). Secondly, the arrangement of the ommatidial 

 centres in reference to the hexagonal plan is more symmetrical in the 

 young lobster than in Crangon, where the rows of ommatidia have ap- 

 parently slipped somewhat upon one another so as to resemble more 

 nearly the condition in the adult lobster. 



I have been unable to determine with certainty what occasions the 

 change from the hexagonal to the tetragonal arrangement. Apparently 

 it accompanies an excessive growth on the part of the individual omma- 

 tidia. In the lobster, for instance, the ommatidia rearrange themselves 

 between the times when the young animal is one inch and eight inches 

 long. During this period the ommatidia increase about ten times in 

 length and about five times in breadth. The increase is especially 

 noticeable at their distal ends, and particularly in the cone cells. In 

 young lobsters of one inch in length (Parker, '90 a , Plate IV. Fig. 55), 

 the space between the cones of adjoining ommatidia is considerable ; in 

 adults, it is proportionally very much less (compare Parker, '9()\ Plate I. 

 Fig. 5), and the cones are crowded against one another. Under these 

 conditions, the hexagonal arrangement apparently gives way to the te- 

 tragonal. So far as I am aware, the tetragonal arrangement occurs only 



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