LIMULUS AN ARACHNID, 641 
evidence in favour of the view that Peripatus is a repre- 
sentative of an exceedingly primitive grade of Arthropod 
development, corresponding to a period when the Arthropod 
branch had advanced but little on its special lines of differ- 
entiation. 
.At the same time Peripatus is specialised and adapted to 
a terrestrial mode of life. It possesses no remnants of bran- 
chial organs but a peculiar tracheal system, air being admitted 
to the fine vessels formed by its vasifactive tissue through 
irregularly scattered gland-like pits of the integument. 
Its specialisation as a terrestical organism has, it is im- 
possible to doubt, affected in Peripatus the locomotor appen- 
dages also, so that much important information is wanting 
to us, which, on the contrary, an aquatic form belonging to 
the phase of development indicated by the eyes, nerve-cords, 
nephridia, and gnathites of Peripatus, could have furnished. 
It appears to me that we have no such aquatic represen- 
tative form, and that Peripatus stands as a specialised ter- 
restrial off-shoot at a much lower point in the Arthropod 
family-tree than that at which we find outgrowths of exist- 
ing branchiate Arthropoda. 
The antenne of Peripatus probably are identical with the 
similar organs of Chetopoda (cf. Spio and Phyllocheto- 
pterus), and are zoé originally post-oral appendages which 
have become preoral by adaptational shifting of the oral aper- 
ture, but are actual lobes or processes of the primitive prosto- 
mium, like the tentacles on the head of a snail, and inner- 
vated by the archicerebrum or original prostomial ganglion. 
In the interval between the giving off of Peripatus and 
the production of the Phyllopod-lke ancestors of the Crus- 
tacea from the aquatic Pro-Arthiopoda, a vast change had 
to be effected in regard to appendages as well as in the 
fusing of the nerve-cords, abolition of nephridia, produc- 
tion of a compound eye, striation of muscular tissue, &c. 
The prostomial antennz disappeared and their place was 
taken first by one, then by two pairs of post-oral appendages, 
which gradually acquired a pre-oral position as actually 
occurs in their individual growth in the embryo at the 
present day; eventually the simple prostomial ganglion 
(archicerebrum) became complicated by the fusion with it 
of ganglionic material proper to the two shifting appen- 
dages, though in the existing Phyllopod Apus it still retains 
its original purity and independence. 
The other appendages probably all acquired at one stage 
a development of their basal portion which served as an 
accessory organ for the purpose of bringing food to the 
