ARTHROPODA 27 
1S5) 
continuous with the body only in the region of the head. This 
mantle is strengthened by five calcareous plates, the carina 
median and dorsal, two scwta near the stalk, and two terga at 
the free end. Owing to the disposition of these plates, the 
mantle can only be opened along the ventral surface; when 
opened the biramous thoracic legs can be protruded, and their 
movement sets up currents in the water which bring food to 
the mouth. In Pollicipes and Scalpellum, genera in which the 
stalk is smaller than in Lepas, there are a number of triangular 
secondary calcareous plates which arise round the top of the 
stalk and form a kind of collar round the body of the animal. 
Some of these secondary plates are still further developed in 
Fic. 162.—View of Bal- 
anus  tintinnabulum 
after removal of the 
right shell. From 
Leuckart and Nitsche. 
After Darwin. 
Tergum. 
Scutum. 
Outer shell in section. 
Antennae, 
Ovary. 
Oviduct. 
Opening of oviduct. 
Labrum. 
Adductor muscle. 
Depressor muscle of 
tergum. 
11. Depressor muscle of 
scutum. 
12. Mantle cavity. 
Bw OM ES So hort 
& §2 ge 
Balanus, which is without a stalk, and here they form a 
calcareous tube consisting of six pieces, the median dorsal one 
being regarded as equivalent to the carina (Fig. 162). Within 
this tube the body of the animal lies protected by four plates, 
the two scuta and two terga. 
The head is not marked off from the thorax, except by the 
position of the appendages, and the abdomen is reduced to a 
short stump, at the end of which the anus is situated. 
Appendages of Lepas. 
1. Antennules. 4. 1st mavxillae. 
2. Antennae (lost in larval life). 5. 2nd 4 
3. Mandible. 6-11. 6 pairs of thoracic limbs. 
18 
