42 DISCOVERY REPORTS 



tend to moult into larvae, which again have seven spines, while the larger ones moult 

 directly into five-spined forms. This is shown in Table XIII for two samples of seven- 

 spined larvae. 



Table XIII. Length frequencies of larvae with seven terminal spines, arranged according 

 to the number of spines which will be present in the following stage 



There is a considerable overlap in the size ranges of the two groups of larvae in which 

 the number of spines in the succeeding stage can be determined. The normal reduction 

 of the number of terminal spines proceeds by the disappearance of the two outermost, 

 and larvae having six spines should be regarded as exceptional. 



The nine larvae mentioned above, when arranged in order of increasing development 

 of the appendages (Fig. 8), furnished a rough scale which was used in the analyses of 

 the samples of seven-spined larvae recorded in Tables XIV and XV. These samples were 

 examined to discover what correspondence, if any, exists between size, degree of de- 

 velopment and the numbers of terminal spines in the teison subsequent to moulting. 

 The letters a toj in the columns of the analyses indicate the form of appendage of the 

 nine larvae which that of the larva in the sample most closely resembled, a being the 

 least and j the most developed. 



In Table XIV the results of an examination of thirty-one larvae are given. It is seen 

 that the rostrum, although it may have a spine in quite small individuals, is in these 

 more often without, whereas in larger larvae there is a greater proportion with a spinous 

 rostral plate. The antennular flagellum increases in length with increasing size of larva. 

 The degree of development of the mandibular palp, ist thoracic leg and teison within 

 broad limits also correspond with the size of the larva. Here again the fact is demon- 

 strated that smaller larvae moult into seven-spined forms, the larger into five-spined 

 forms, but that there is a great amount of overlapping in size and degree of development. 



Table XV shows a similar analysis of larvae from St. 639. The average size of the 

 larvae here is much less than at the previous station cited, and in correlation with this, 

 development is much less advanced ; nearly all the larvae indicate that in the following 

 stage they will be seven-spined. 



