i896. NOTES AND COMMENTS. 85 



appendages in the Nauplius is almost exactly the same for all species. 

 In the various forms examined he found the same number of joints on 

 corresponding branches or basal pieces, and the same number of 

 bristles, spines, or teeth on each of the joints, the agreement extend- 

 ing even to the distinction between simple and plumose spines and 

 bristles. That earlier descriptions did not entirely confirm this 

 conclusion was the less surprising, since sometimes Mr. Groom's own 

 failed to do so ; but he had only to see a difference in two of his sketches, 

 " to discover upon re-examination of the object that a mistake had 

 been made in one or other." From this perfect similarity he infers 

 " that the character of the appendages is a primitive one, actually 

 possessed by the common ancestor of the thoracica at some stage in 

 its life-history." But this common ancestor is of ancient lineage, for 

 "a careful study of the thoracic Cirripedes indicates that the Balanids,, 

 on the one hand, have probably diverged from Pollicipes, and the 

 majority of the Lepads, on the other, from Scalpellum," these two 

 genera being connected, as Darwin and Hoek (and more recently 

 Aurivillius) have shown, by various intermediate forms. Hence it 

 may well be said that the " permanence of such minute cha- 

 racters as the arrangement of the bristles on the appendages for 

 the vast time represented by the Tertiary, Cretaceous, and probably, 

 at least, part of the Jurassic periods, is highly remarkable, and well 

 shows the slow rate of evolution which may take place in so highly 

 specialised a group." 



It is perplexing, however, to find that Professor Chun, though 

 expressing the highest appreciation of Mr. Groom's work, by no 

 means corroborates his opinion as to any precise agreement in the 

 number and armature of the joints of the appendages in different 

 Nauplii. There is little room for supposing that Professor Chun has 

 given a wrong account of the facts under his own observation, so that, 

 unless the appendages vary at different stages of nauplian life, it is 

 likely that Mr. Groom's rule may have a less extensive application 

 than he was led to expect. 



Before leaving the subject of embryonic agreement, it will be 

 worth while to quote the following passage from Dr. McMurrich : — 



"The regularity of the entire process of growth of the meta- 

 naupliar region of the Isopods is most remarkable, and the more one 

 studies it the greater is the wonder it excites. The regular rows of 

 ectoderm and mesoderm cells are wonderful in themselves, and when 

 there is added a more or less definite number of rows for all the 

 species, we see that we are dealing with laws of growth which are at 

 present far beyond our powers of explanation. It is true that the 

 number of ectodermal teloblasts is not always quite constant, though 

 approximately so, but it is exceedingly interesting to find that where, 

 as in Aselbis, they can be traced back to their earliest differentiation, 

 there is a definite number of them — namely, eleven. And this 

 definiteness of number is not confined to the Isopods, but is found 

 also in Mysis (Bergh, 1893). ^^ regards the mesoblast, however, the 

 number is more constant, eight, and eight only, occurring in Cymothoa,. 



