380 GEOLOGICAL SURVEY OF THE TEERITOEIES. 



in Apus should be regarded as the first abdominal pair or not must, it 

 seems to us, reuaaiu an open question ; there seems, however, to be no 

 other line of demarkatiou in the family which this genus represents. 



The telson. — This portion of the abdomen, sometimes called "post- 

 abdomen," is large and w^ell differentiated in the two lower families, 

 especially in Estheria — where it is compressed, high, armed above with 

 numerous spines, and bearing below a pair of modified caudal a]> 

 pendages which we shall consider under the head of the appendages. 



We will rapidly recall the salient points in the form of the telson in 

 the three families of Phyllopods. In Limnetis the telson is much as in 

 some OkcZocera, being small, without teeth along the upper edge, though 

 still bearing the pair of dorsal filaments (see £«g. 4, in text, and Plate 

 I, fig. 0). These are also present in certain Cladocera, Daphnia, Bosmina^ 

 &c., and in several genera aMied to Baphnia, Alona, Pleuroxus, &c., the 

 upper edge is dentate or spinedj this with other features in Limnetis 

 shows that the Phyllopods have probably descended from Cladocera-like 

 ancestors. In Estheria and Limnadia the telson is large and densely 

 spined along the upper edge. The large spiny telson is probably of 

 use to aid the animal in pushing itself through submerged dense vege- 

 tation, for all that portion of the body which can be thrust out between 

 the valves is armed with stout spines, whereas in Limnetis, only the 

 telson can project beyond the edge of the carapace valves. 



In Apus the telson is nearly cylindrical, short and small, and flattened 

 from above downwards, and is without much functional value, though 

 the cercopods are of use in swimming; but in Lepidurus it is produced 

 into a long spatulate portion like a beaver's tail, and which must give 

 it an advantage over Apus in extricating itself from muddy places. 



In the Branchipodidm the telson assumes the form of a simple segment, 

 cylindrical, soft, unarmed, b.ut in Thamnocephalus becoming very broad 

 and flattened into a lateral fin-like expansion and without any caudal 

 appendages, which are always present in the other genera of the family. 



What we call the telson, and which is simply the last abdominal seg- 

 ment, is called by Gerstaecker, and we suppose earlier authors, the "post- 

 abdomen." Some authors, the most recent, Gerstaecker in Bronn's 

 Classen und Ordnungen,&c., speak of the "abdomen" and "postabdo- 

 men " in the Branchipodidcc, but they do not state where the tibdomen 

 begins. The term " postabdomen " is applied to the last eight [Artemia] 

 or nine [Branchipus, &c.) segments of the body {uromeres), but we see 

 no good reason for not regarding these segments as forming a true ab- 

 domen (urosome), the first segment, or ninth from the end, in Branchi- 

 pus bearing the external reproductive organs. We really see no need 

 of employing the term "postabdomen" in speaking of any Branchiiiod, 

 nor in fact, so far as we are aware, does it have any special significance 

 in other groups. We here consider the so-called " postabdomen 'i. of the 

 Limnadiadce and Apodidw as the telson, and the homologue of the telson 

 in the macrurous Becapoda. 



The eyes. — There are in the lower Phyllopoda but a single pair of com- 

 pound or facetted eyes, but in the BrmicMpodidce the simple, unpaired 

 eye of the larva is retained. In the Limnadiadm these are sessile ; in 

 the BrancMpodidcB they are stalked. The structure of the eye of Apus 

 cancriformis and Branchipus (species not named) has been described 

 and in part figured by Grenacher in his great work "Untersuchungen 

 liber das Sehorgan der Arthropoden " (1879). The eye of Branchipus 

 stagnalis has previously been investigated by Leydig in 1851 ; and that 

 of Estheria californica by Lenz in 1876. The eye of Limnetis has appar- 

 ently not been investigated, and our own observations on it are but 



